Optimized Particles Size Distribution Lightweight Cement at Low Temperatures: Case Study from Eastern Siberia, Russia

Limited information is available on the effects of pressure on the set properties of cements with respect to commonly used additives. This report presents compressive strength data on cement systems presents compressive strength data on cement systems cured at temperatures of 170 degrees to 260 degrees F and at pressures of 3,000 to 10,200 psi. Of the cement systems studied, those designed for a specific bottom hole static temperature (BHST) show little change in compressive strength with increased curing pressure at BHST. However, some systems, cured at pressure at BHST. However, some systems, cured at temperatures lower than BHST, gave increased or decreased compressive strength with increased curing pressure. The use of curing pressures simulating more closely those actually found in oil or gas wells gives a better understanding of additive performances and a more realistic waiting on cement (WOC) time for cement systems. Introduction Several authors have shown that the effects of curing pressure above 2,000 psi are negligible on the development of 24-hour compressive strengths on neat cements at a curing temperature of 200 degrees F. However, the development of 12-hour compressive strengths on oil and gas well cement systems of varying retarder content, cured at different bottom hole circulating temperatures (BHCT), has been shown to increase with increased curing pressure. No other, or only limited information, is available concerning the effects of curing pressure on the development of compressive strength on cement systems with respect to various additives and combinations of additives. Today in numerous wells, cement systems are placed under varying pressures in excess of 3,000 psi. Therefore, in order to improve the design of cement systems for these wells, an understanding based on experimentation of the effects of these higher curing pressures on the performance of additives is needed. performance of additives is needed. This paper examines the effects of curing pressures of 3,000 psi to 10,200 psi on the 8, 12 pressures of 3,000 psi to 10,200 psi on the 8, 12 and 24-hour compressive strengths of cement system with various additives, cured at 170 degrees to 260 degrees F. The main purpose of this work is to study the compressive strengths of cement systems at temperatures and pressures that could exist at the top and bottom of a cement column under simulated well conditions. Discussion The number of various types of additives used in any given cement job is directly related to the depth, temperature, and type of formation encountered in an oil or gas well. With the large variety of additives in existence and the wide range of well conditions possible, some limitations had to be imposed on what was to be studied in this preliminary investigation. The cements examined were preliminary investigation. The cements examined were limited to a class H, class G and a 50:50 fly ash: class H. The additives examined were limited to a lignosulfonate retarder, a cellulose fluid loss additive, bentonite, and salt. The well conditions under which samples were cured were based mostly on those possible in a 12,000 ft. well. The specific cement systems examined are listed in Table I with the mix water, density, and thickening times. All the cement systems containing additives, except the salt systems, were designed for placement in a 12,000 ft well with a BHST of 260 degrees F. These systems all had 4–6 hours thickening time at a BHCT of 197 degrees F. The systems were all cured at 170 degrees, 200 degrees, 230 degrees, and 260 degrees F under various pressures for 24-hour compressive strength determinations with the lower temperatures used to simulate the tops of various length liner cement jobs. The salt systems were examined only at 170 degrees F because this was representative of a temperature at which salt may be used alone. The 30 degrees F intervals in curing temperatures constitute 2,000 ft depth changes in wells with a 1.5 degrees F/100 ft depth temperature gradient.

This paper outlines an innovative approach to optimize workover re-entry wells eliminating the liner and tie-back operations by addressing challenges that had placed this strategy as not suitable in the previous history of the onshore operations around the area. The main concern deemed impossible was cementing a 7″ Production Casing from the landing point to the surface safely and successfully. This strategy is essential for the development of the field and to efficiently deliver the well count on time. The project started by designing and selecting the casing materials, required auxiliary equipment and mitigating all the risks associated with the equivalent circulating density (ECD) during drilling and cementing and refining operational procedures, the projected provided a more operationally efficient well, in a shorter time, lower cost and a more integral well with not weak points as the tie-back connections and the stage tool ports. This technique eliminates the need for running and cementing both a 7″ liner and a tieback, streamlining the process while ensuring uncompromised well isolation. Hydraulic simulation, drawing from available field data, was employed to assess acceptable equivalent circulating densities (ECDs) and pressures without risking losses. These simulations guided the refinement of cement slurry designs. A specialized lightweight cement system was suggested to address ECD margin limitations and challenges linked with conventional lightweight cement. Additionally, an expandable gas-tight system was introduced to prevent zonal communication and micro annulus in between the different reservoirs. Further enhancements included the introduction of sealing/bridging spacer to ensure effective mud removal along with the optimization of pumping rates to minimize losses during displacement. Optimized pumping rates during drilling, well bore cleaning and conditioning and cementing operations were guided by simulation data to ensure that dynamic pressures remained safely below fracture pressure thresholds. Additionally, it was decided to include higher density mud to assist in reducing surface pressures during the displacement. The utilization of a specialized lightweight slurry proved effective in decreasing hydrostatic pressure of the fluid column, thereby preventing lost circulation incidents and guaranteeing a positive indication of cement return to the surface. The drilling fluids conditioning, the spacers train and the exceptional rheology of the slurry facilitated improved mud displacement and enhanced cement coverage during placement. Furthermore, a higher solid volume fraction (SVF) in the cement contributed to reduced porosity and permeability, ensuring long-term zonal isolation and casing protection, thereby minimizing maintenance requirements and extending the well's lifespan, especially in critical reservoirs. The expandable property of the set cement mitigates for any potential micro annulus development, helping to avoid sustained casing pressure (SCP) complications throughout the well's operational life, thus reducing the need for further remedial cementing operations. Moreover, a specialized spacer system efficiently aided in mud removal and demonstrated excellent bonding, as confirmed by cement evaluation logs (CEL) conducted across the 7″ casing. This comprehensive design approach effectively eliminated additional operational costs and rig time by circumventing the necessity for further remedial interventions. This paper aims to showcase the utilization of 7″ production casing in conjunction with specialized cement design as an efficient substitute for conventional liners and tiebacks. The goal is to reduce rig costs and save time during workover re-entry wells, while also minimizing the need for future costly remedial works.

Cement Systems require conformance of specific parameters for the longevity of the system placed as a sheath. Challenges of the system design are exacerbated considering the environment for placement of the sheath. HPHT cement systems require special design attention, and the authors propose to share one such case history in designing of the HPHT system for Eastern India Offshore. HPHT cement systems require not only modified testing procedures, but products also. HPHT cement system products are required to withstand BHST temperatures for the life of the well without considerably altering the slurry system properties. Modified test procedures and products requirements for slurry system bring additional logistical challenges like preparation and availability of right cement blend for testing and preparing the slurry system for the application. Considering the temperature gradient of 2.23°F/100ft, bottom hole static temperature (BHST) for the 7" liner job was 467°F. The mud weight for the section was planned between 14-16 ppg (1.67 to 1.9 sg). Computed BHCT alone was considered inadequate and determination of the hot spot in the wellbore was required. Computerized simulator was used for the purpose. Results show that the hot spot temperature predicted by the simulator was higher than the computed BHCT and the depth of the hot spot was shallower than TVD as normally anticipated. The simulator runs are recommended for determination of hot spot for HPHT testing of cement systems in particular. The simulator considers effects of various fluids pumped and their contact times along with the heat capacities during the application. Product choice for preparation of the Cement systems used earlier for the high temperatures encountered in Western India offshore were reviewed and preliminary testing was carried out on the basis of the prognosed temperature. However predictions from the temperature simulator yielded higher temperatures for the East coast well. This required a different product set for the cement system design. The authors feel that the review of this case history and the design challenges at this technology meet shall potentially benefit the design engineers in understanding and enhancing their knowledge about design practices for HPHT cement systems.

Ultra-deep wells (maximum measured depth of 8000 m) located in the Tarim Kuqa foreland oil field in western China pose significant challenges to well integrity because of the high temperature (maximum bottom hole static temperature of 180°C), high pressure (maximum 145 MPa), and a deeply buried, thick, salt-gypsum formation (maximum measured depth of 7800 m). An engineering cementing approach with continuous improvements has been applied in the cementing operations in over 200 wells from 2008 to 2015. The ultra-deep casing program was designed to meet the complex drilling and high-pressure completion requirements. Meanwhile, fit-for-purpose cement systems have been developed to seal the long, high-pressure interval of the salt-gypsum formation. In addition, a procedure on spacer optimization and evaluation has been established to optimize target zone cementing in oil-base mud. Tailored cementing procedures from well preparation to post-job remedial work were developed to meet the severe lost circulation challenge caused by the narrow pore-fracture pressure window in the reservoir. Since 2008, various problems related to well integrity have been encountered in the field, including casing leaks, incomplete zonal isolation post cementing and sustained casing pressure (SCP) during the exploration and development activities. Substantial local operational practices have been accumulated to solve these intractable problems through the 1200 primary and remedial cementing operations undertaken to the present. A comprehensive pool of cement slurry formulations has been established with optimized properties including extreme high density with good pump-ability, saturated salinity resistance, and cement setting under an extended temperature difference to meet the various cementing requirements resulting from complex and extreme downhole conditions. A design-execution-evaluation (DEE) cycle of cementing operation has been implemented to achieve continuous improvements and shorten the learning curve in the face of the new challenges continuously encountered during the extended exploration activity of this region. The annual average percentage of qualified cementing jobs has been enhanced steadily from an initial 55% in 2008 to the present 73%. The nature of this ultra-deep reservoir limited the application of lessons learned from other oil fields. A series of cementing practices developed empirically to address the challenges of this field have been documented and optimized over the years. They have proven to be successful in the field and form the foundation of designs going forward.

In 2018, an operator in Malaysia completed a sidetrack campaign consisting of injector wells. These wells were planned for maximum productivity via sustainable wellbore zonal isolation. The presence of Carbon Dioxide (CO2) in these wells elevated concern about the zonal isolation of cement across the interval. Moreover, for an injector well, the cement must exhibit resilient properties by design of enhanced mechanical properties to provide long-term isolation based on a cyclic wellbore. An advanced slurry system was designed that enabled the set cement to manifest superior properties in three parameters—corrosion resistance against CO2, flexibility against wellbore stress changes, and expansion to mitigate microannuli. The design of the slag-based flexible cement system with expanding additive (slag-flex) considered all three parameters in the fit-for-purpose application of a resilient and flexible expansive cement system in a CO2-rich well. The system’s mechanical properties, such as Young’s Modulus, Poisson’s Ratio, and tensile strength, were verified with laboratory-scale testing and validation against stress analysis software to confirm on the resilient and flexible properties. The laboratory testing result demonstrated the improved properties of the system, including high tensile strength and low Young’s modulus. Furthermore, the reduced water content of the system decreases the permeability of set cement and thus increases resistance towards corrosive substance such as CO2. For certain cases in the past, two separate slurry systems had to be designed—a lead slurry with CO2-resistant properties and a tail slurry with flexible and resilient properties. Often, several issues arose from this practice, including complex logistics due to cement silo blend arrangement and complexity during job execution. Hence, this new system presents a novel idea and methodology that will deliver value to the oilfield industry by integrating CO2 resistance, flexibility and expansion properties in a single slurry system. The system was successfully pumped in wells in Malaysia; no sustained casing pressure has been recorded to date, and wells have been delivered to their intended zonal isolation requirements without compromising well design and overall integrity. This is an innovative application of this type of cement system in the region, and the long-term zonal isolation and well integrity assurance in these and future wells have the potential to save millions of dollars in remedial work. The cement system is currently recognized as the default technology for CO2-rich injector wells in Malaysia.

A major operator on the Caspian Turkmen shelf has started to encounter sustained casing pressures (SCP) attributable to insufficient isolation across a hydrocarbon gas zone, due to downhole stresses and other contributing factors. Enhanced placement techniques of conventional cements failed to prevent SCP, confirming the requirement for an alternative cement system that can withstand anticipated stresses and resolve this challenge. An innovative and cost-effective solution was applied and successfully solved the SCP challenge due to its unique self-healing properties. If cracks or microannuli occur and hydrocarbons reach the cement, the system has the capability to repair itself, restoring integrity of the cement sheath without external intervention. The cement system is placed conventionally in the annulus across or above the hydrocarbon-bearing formation. It then acts as a pressure seal, expanding to accommodate downhole changes and healing if any hydrocarbon reaches it. This technology has been used in four wells in the field with excellent results. Two wells were used to demonstrate the capabilities of the self-healing cement as a lead cement slurry, which created a cap over the pay zones. The self-healing cement was designed with low Young's modulus for optimum flexibility. To minimize the risk of set cement integrity failure due to microannuli or microdebonding from chemical shrinkage after setting, linear expansion up to 1.2% was incorporated into the design. After cementing, the wells were intentionally exposed to a sequence of high-pressure tests, which induced annular pressures in the wells. However, because of the self-repair capability of this cement, isolation and integrity were effectively restored in the two wells within 1 to 2 weeks without external intervention. As a result, the self-healing cement technology has become the standard for the field for all future wells, and the operator plans to extend the self-healing cement technology to other fields with similar challenges. This paper clearly demonstrates successful casing pressure remediation without intervention by engineering a flexible, self-healing cement system. The design strategy, execution, evaluation, and results for two wells are discussed in detail and will help to guide future engineering and operations around the world.

A major Exploration and Production (E & P) company was posed with a challenge of sustained casing pressure in over 85% of the wells drilled in one of the fields in the Offshore Niger Delta. Sustained casing pressure occurs when the annular seal is damaged and a path is created for the formation pressure to reach the surface, and the pressure in the annulus rebuilds after being bled down. When cementing in such an environment, operators have the following objectives: Placing a cement sheath with increased capability to survive future downhole stress events.Achieving effective mud removal.Gaining the benefits of a react and respond sheath design.Providing a long productive life for the wellbore. To develop a cement system that would achieve and maintain isolation throughout the life of the well, an advanced cement technology was required. This advanced cement technology involved Finite Element Analysis (FEA) to model the effects of stresses from well operations on the cement sheath. This analysis takes into consideration well configuration, formation properties, casing properties, cement properties and operational loads and predicts the remaining capacities of the cement sheath. The remaining capacity is used to measure how much a cement sheath is stressed. After simulating the effect of downhole stresses on a cement sheath, a mechanically enhanced cement slurry was designed to meet the recommendations of the model and provide a cement sheath with improved remaining capacity. This paper presents the successful mechanically enhanced slurry design, job design, planning and execution on the production section of the well after a Finite Element Analysis was conducted. Post job conditions such as good cement bonds across cemented areas, well production without inter-zonal communication and no annular pressure build up in over 8 years have proven the success of the design and the procedure implemented in these challenging well bore environments. The success of this well has been applied to two additional wells.

A good primary cementing job governs in a great part the producing performance of a well. Successful zonal isolation, which is the main objective of any cementing job, primarily depends on the right cement design. The resin-based cement system, which is a relatively new technology within the oil industry has the potential to replace conventional cement in critical primary cementing applications. This paper describes the lab-testing and field deployment of the resin-based cement systems. The resin-based cement systems were deployed in those well sections where a potential high-pressure influx was expected. The resin-based cement system, which was placed as a tail slurry was designed to have better mechanical properties as compared to the conventional cement systems. The paper describes the process used to get the right resin-based cement slurry design and how its application was important to the success of the cementing jobs. The cement job was executed successfully and met all the zonal-isolation objectives. The resin-based cement's increased shear bond strength and better mechanical properties were deemed to be instrumental in providing a reliable barrier that would thwart any future issues arising due to sustained casing pressure (SCP). This paper describes the required lab-testing, lab-evaluation, and the successful field deployment of the resin-based cement systems.

As part of well integrity and safety campaigns in offshore fields, downhole Temperature surveys were obtained as record of temperature values measured with respect to depth steps (logs) in a well. The downhole condition for each well is an essential factor to determine the well status and to prevent a serious reservoir damage if there is a cross flow phenomena. Therefore, temperatures measurement at varies depths were used to establish the temperature profile which is reflecting the downhole condition. Several oil wells are subject to be evaluated through the campaigns in terms of downhole integrity especially casing leaks problems. Due to long period of shutdown for the field, close monitoring surveys with more focus on the aged wells are required to avoid flow behind pipe and cross flow phenomena, which will lead to the formation damage and production losses. The applied approach which is related to Well Integrity Management (WIM) started by creating a baseline in terms of temperatures profile for each well at cost effective time-lapse. The reference is a geothermal gradient log recorded in shut-in condition. Slickline operations were performed to execute the bottomhole temperature surveys. The temperature profile interpretation added great value in terms of confirming the wellbore integrity and detecting anomalies such as a sustained casing pressure and casing leaks. Effectiveness Well Integrity Management (WIM) included the methodology of monitoring, detecting, and localizing leaks phenomena across shallow aquifers was used to extend the well operability life. The approach was implemented on several field cases in order to detect casing leaks in offshore oil wells. A capture of temperature anomalies were found with a clear deviation from the baseline gradient. Based on the evaluation results, many of anomalies were related to the entry of fluids into the borehole. However, there were some cases indicated that the fluid flow was upward. It is noted that the temperature was affected by the type of occupied fluid into the outside casing and by the type of movements. As a result, the temperature profile was sensitive to not only the borehole condition but also the formation type and the casing-formation annulus. The paper covered field wells completions evaluated and interpreted their temperature profiles to capture the temperatures anomalies leading to casing leak, flow behind pipe, and a cross flow phenomena. The approach applied further investigation by integrating technique with other integrity surveillance logs. In addition, the results from the workover operation with the remedial actions shared in order to validate the findings.

An efficient management of the newborn dairy calf around birth results in reduced rearing costs and increased future milk yield. To maintain young calves healthy and maximize performance and profit it is important to create an environment that maintain low levels of stress. Low environment temperature directly affects newborn and young calves’ performance, health and survival due hypothermia. For that reason, the adaptation of the dairy calf to the cold environment is important, with body responses of primary importance. However, some strategies may also be adopted to reduce cold stress induced by heat loss. There are several ways to aid the young calf in low environmental temperature, starting immediately after birth with colostrum feeding. Besides that, factors such as adequate housing, management and nutrition, result in significantly decrease morbidity and mortality, and similar growth rates as calves in the zone of thermoneutrality. The calf physiology adaptation to cope with cold stress and the positive effect of feeding systems, resulting in greater heat production, health, and performance will be approach in this literature review.

Lightweight cement systems are used in the weak intervals of petroleum wells. Sodium bentonite is used as an extender in lightweight oil-well cement systems as it prevents excess water and sedimentation of particles, thereby ensuring the formation of homogenous and stable cement sheaths. The extending ability of sodium bentonite is enhanced when prehydrated. However, the optimum bentonite prehydration time and its effect on the stability of lightweight cement systems have not been well established. The objective of this study is to investigate the optimum sodium bentonite prehydration time and correlate it to the stability of lightweight oil-well cement systems. Bentonite suspensions were prepared by vigorous preshearing at 12000 rpm for 5 minutes, followed by aging times of 0, 30, 60, and 120 minutes. The swelling behavior of bentonite was investigated using a laser particle size analyzer. The Herschel-Bulkley model was used to determine the rheological parameters of the experimentally measured shear stress vs. shear rate data of the aged suspensions. The effect of calcium chloride salt on aged bentonite suspensions was investigated. Density measurements and pore space analysis with the nuclear magnetic resonance (NMR) technique were used to investigate the homogeneity of cement-based cores. It was observed that bentonite swells with time and, after 30 minutes, the swelling is insignificant; however, the swelling property did not have any observed impact on the properties of cement systems designed with the bentonite aged at different times. In general, all the lightweight cement slurries exhibited similar properties, in terms of rheology, stability, and homogeneity, regardless of the bentonite prehydration time. These findings indicate that aging bentonite suspension after vigorous preshearing in lightweight cement design is unnecessary and would only contribute to nonproductive time.

Chlorine disinfection has been reported to be ineffective in controlling RO membrane biofouling in some projects. Feed water temperature is a crucial factor in the formation of RO membrane biofouling. It has a positive impact on the wide application of the RO process to ascertain whether chlorine disinfection can alleviate the membrane biofouling at low temperatures. In this study, the effects of chlorination on the RO membrane biofouling at low feed water temperature (10 °C) were investigated by a lab-scale RO apparatus. The final normalized flux was 0.33 and 0.29 with and without chlorination, respectively. According to the normalized flux decline curve, chlorination could not alleviate the RO membrane fouling at low temperature. Based on the intermediate blocking model, chlorination increased the membrane fouling potential of the feed water. At low temperature, the biofilm on the membrane with chlorination was thinner and denser than that without chlorination. In addition, the membrane with chlorination contained more foulants and dissolved organic matter than that without chlorination. Chlorination failed to continuously prevent bacteria accumulation on RO membrane at low temperature, but screened out bacteria that were potentially more suitable for the low-temperature membrane environment.

The important role of improving the activities of all law enforcement agencies depends on the scientific level of their organization. An essential part of the methodology for the investigation of any crime is the element devoted to the specifics of investigative tactics, a special place among which is occupied by an investigative examination of the scene of an incident, from which an investigation usually begins. Investigative actions (in particular, investigative examination) carried out under low air temperature conditions have their own significant features that require separate study. This paper deals with the peculiarities of carrying out a type of procedural investigative action, such as examination in the Arctic conditions (low air temperature), gives practical recommendations to law enforcement officers on how to act in conditions related to adverse weather situation (low temperatures, shortened daylight), deals with the peculiarities of work in these conditions with traces of hands, feet, vehicles, burglary; special attention is paid to the peculiarities of working with odor and shooting traces; special attention is paid to the peculiarities of work at the scene of corpse location and of work with blood traces. General scientific methods were used as the research method - analysis, synthesis, determination of cause-effect relation of the phenomena in conditions of low temperature, and also the philosophical law which assumes differentiation of scientific knowledge about scientific grounds of tactics of procedural investigative actions in combination with the necessity of integration of the received knowledge with other sciences (logic, criminal procedure law, medicine).

Research on flow pattern of low temperature lubrication flow field of rotating disk based on MPS method

국내에 자생하는 참억새(M. sinensis)와 물억새(M. sacchariflorus)의 잎과 지하경 조직 EST로부터 유전자의 전사를 조절하는 전사요소 탐색하여 저온에 반응하는 유전자를 분리하고 난지형 잔디에서 저온 반응의 차이를 알아보기 위하여 본 연구를 실시하였다. 분석 결과 탐색된 전사조절 요소의 종류는 총 50 종류로 나타났고, 그 중 WRKY family에 속하는 EST 절편이 226개로 가장 많이 발견되었다(9.6%). 그 중 억새 WRKY family를 기능이 밝혀진 다른 작물의 WRKY 유전자들과 비교 검색한 결과 약 80개의 억새 isotig가 저온에 반응하여 발현이 유도 또는 억제되는 것으로 나타났다. 그 중 애기장대와 벼에서 저온 처리 후 발현이 증가되는 것으로 알려진 억새의 MSIR7180_ WRKY4 유전자를 대상으로 그 발현양상을 조사한 결과 버뮤다그래스는 비교구에서와 비슷하게 처리기간 동안 높은 유전자의 발현을 보였고, 금잔디(Z. matrella)간의 교배를 통해 얻어진 세밀 품종은 처리기간 내내 발현이 미약하였다. St. Augustinegrass는 3일 동안은 비교구와 큰 차이가 없다가 5일째부터 발현이 증가하였고, Seashore paspalum은 처리 초기에 발현이 높다가 처리가 진행되면서 약해지는 경향을 나타냈다. 이 결과로 미루어 볼 때 버뮤다그래스와 St. Augustine grass는 저온 반응성이 신속하여 휴면을 준비하지만 금잔디와 Seashore paspalum은 휴면 돌입이 늦어 녹색이 상대적으로 늦게까지 유지되는 것으로 판단되어 저온 적응을 위한 또 다른 환경요인의 작용이 있을 것으로 여겨진다. 녹색 기간이 길고 내한성이 높은 품종의 개발을 위해서는 더 많은 유전자의 종합적인 고찰과 판단이 요구된다. Whole genome transcriptomes from Miscanthus species were sequenced and analyzed, which provided 50 different types of transcription factor (TF) involving various developmental processes or environmental stresses. Among the explored TF, WRKY gene family was the major type and one of the WRKY genes, MSIR7180_WRKY4, induced under low temperature environment was selected to investigate how the Miscanthus-originated MSIR7180_WRKY4 TF responds when exposed to low temperature in four warm-season turfgrasses (Z. matrella 'Semil', bermudagrass, St. Augustinegrass, and seashore paspalum). The MSIR7180_WRKY4 was expressed higher during low temperature period in Bermudagrass, but the expression was enhanced in St. Augustinegrass. In contrast, the gene in 'Semil' cultivar was barely expressed and relatively less expressed, but repressed gradually in seashore paspalum, which seems to allow two turfgrasses stay-green longer in the fall season. The results indicate that bermudagrass and St. Augustinegrass adapt to low temperature quickly, but relative tolerance to low or cold temperature at the molecular level needs to be further investigated at different physiological stages and the corresponding genes systematically.