Thermal Well Screen Plugging: Revisiting an Old Challenge, Part 1: Role of Drilling Fluids in Screen Impairment

During drilling operations, mud is typically circulated at pressures above the reservoir pressure, and this causes solid particles to deposit along the wellbore creating a mud cake, while the mud filtrate can invade into the formation creating a damaged zone. Near wellbore damage can result in reduced permeability, increased skin and thereby lower productivity of the reservoir, while thick mud cakes can cause issues during drilling including stuck pipe, high torque as well as excessive swab and surge pressures during tripping. These can be prevented by designing the drilling fluid to reduce the quantity of mud filtrate that enters the formation. To facilitate the drilling process, the filter cake that forms should also be thin, uniformly distributed, and stabilized. New techniques such as the use of nanoparticles in the drilling mud, as well as ultrasonication of drilling mud before circulation has demonstrated potential in reducing the filtrate loss into the productive zones, while also optimizing the mud cake formed. This current work studies the effect of adding silicon dioxide (SiO2) particles with spherical morphology and multi-walled carbon nanotubes (MW-CNT) with cylindrical morphology into a water-based drilling fluid sample to improve its filtration characteristics. Systematic ultrasonication of the drilling mud was also conducted to optimize filter cake properties, with the aim to reduce their thickness, while also reducing filtrate loss into the formation. To assess the fluid loss, measurements are made of the filtrate volume, the dry and wet weights, and the filter cake thickness. Based on the experimental results, the amount of filtrate/fluid loss can be decreased with the use of nanoparticles. The weight of the filtrate cake is found to have increased with an increase in nanoparticle loading. Based on filter cake thickness, stability, and porosity estimates of the filter cake, low-concentration SiO2-nanomud (at 1wt%) is observed to be a better candidate than MW-CNT-nanomud to control fluid loss and to maintain wellbore stability. Fluid loss is also observed to be reduced by the ultrasonication process, while the filter cake formed by ultrasonicated drilling mud is thinner and less permeable when compared to an unsonicated sample. Reducing fluid loss is a priority in drilling operations to prevent formation damage and novel techniques such as nanoparticles and ultrasonication can provide optimal results for this issue. The addition of nanoparticles is observed to increase the filtrate cake thickness and reduce the filter cake's porosity, thereby reducing fluid loss. Ultrasonication of drilling mud also produced better filter cake with reduced permeability which in turn reduced fluid loss into the formation.

Numerical investigation of non-uniform sand retention behavior in sand screens

Encountering mud cake is a natural consequence of the drilling operation in oil and gas wells. Thick mud cake requires extra drilling efforts to remove it. Thick cake increases filter loss, reduces wellbore space and causes the drill pipe to be prone to get stuck. This study aims at enhancing the performance of the water-based bentonite by using drilling mud hydrophilic, smart nano clay to modify the rheological properties and reduce the thickness of the mud cake. Hydrophilic, smart nano clay is used to enhance water-based mud performance. The content of the bentonite is varied between 2% to 8% by weight of water and the temperature is varied between 25°C to 85°C. The nano clay particle size is increased by 1% starting from 12 to 20 nm in order to modify the rheological properties and, hence reduce the cake. The samples are hydrated, and their pH is adjusted. Viscometer FANN 35SA is used to measure the rheological properties of all samples under standard conditions. The thickness of the cake is measured using MITUTOYO Vernier Caliper. As a matter of fact, by using the nanoparticle water-based mud, it can reduce the mud cake thickness by approximately 56.25%. However, the filtrate volume surprisingly increased by 89% as compared to the collected volume with ordinary clay, determinant coefficients for the base fluid and nanofluid were found at 87% and 96%, respectively, which are close to the best-fitted lines. The average base fluid and nanofluid were found at 0.23657 and 0.12225 inches, respectively. The standard deviation of the base fluid is 0.08229, indicating that approximately 8.2% of the base fluid's data points deviate from the mean. The value of the nanofluid is 0.02363, indicating that approximately 82 percent of the base fluid's data points deviate from the average value. When compared to normal clay, the rheological characteristics, gel strength, and filtrate volume of nano clay illustrated a negative influence on drilling fluid performance and deviate from the API standard specifications. The values for PV, YP and gel strength indicate a high slope trend, however, the filtrate volume collected shows a decreasing slope trend. Results prove that the proposed nanoparticle water-based mud is successful and has reduced the cake thickness by nearly half compared to the ordinary clay. The Hydrophilic smart nano clay introduced here is a new technology. Resources of the material are available in nature and can be obtained with ease and low cost. This investigation outcome is based on solid experimental results and can be implemented in the field without extra efforts or the need for scale-up.

Reported are the results of an experimental programme to investigate the influence of mud composition, pressure and temperature on the properties of filter cakes formed during static filtration. Three water-based muds were tested at pressures up to 4.2 MPa and temperatures up to 93°C. The spurt loss, cumulative fluid loss, cake thickness and porosity were measured and, using a modified static filtration equation, the cake resistance and compressibility were calculated. The results clearly illustrate the highly compressible nature of filter cakes formed from drilling muds. Also shown is the profound influence of barite and temperature on the cake properties and fluid loss. Using expressions developed in the general field of solid-liquid separation for the variation in porosity within the filter cake, the behaviour of highly compressible cakes under pressure has been illustrated.

The X-ray computed tomography method has provided unrivalled data about the characterization and evolution of the internal/external structure of materials by analyzing CTN and non-destructive imaging approach. Applying this method on the appropriate drilling-fluid ingredients plays a significant role in generating proper mud cake quality to stabilize wellbore, and avoid formation damage and filtration loss by preventing drilling fluid invasion into the formation. In this study, smart-water drilling mud containing different concentrations of magnetite nanoparticles (MNPs) was used to assess the filtration loss properties and formation impairment. Conventional static filter press, non-destructive X-ray computed tomography (CT) scan images and high-resolution quantitative measurement of CT number method were used to estimate the filtrate volume and characterize the filter cake layers, hence evaluating the reservoir damage through hundreds of merged images. The CT scan data were combined with the HIPAX and Radiant viewer digital image processing. The variation in CT number of mud cake samples under different concentrations of MNPs and without MNPs concentration were analyzed, and hundreds of 3D images as a cross-sectional profile were used. This paper highlights the importance of MNPs property in terms of minimizing filtration volume and improving mud cake quality and thickness, and hence improving the wellbore stability. From the results, a notable reduction of filtrate drilling mud volume and mud cake thickness to 40.9% and 46.6%, respectively, were recorded for drilling fluids incorporated with 0.92 wt.% of MNPs. However, this study asserts that optimal MNPs should be implemented to guarantee the best filtration property. As confirmed from the results, increasing the MNPs concentration beyond the optimal value (up to 2 wt.%) increased the filtrate volume and mud cake thickness by 3.23 and 33.3%, respectively. CT scan profile images show two layers of mud cake produced from water-based drilling fluids possessing 0.92 wt.% MNPs. The latter concentration was found to be the optimal additive of MNPs as it caused a decrease in filtration volume, mud cake thickness, and pore spaces within the structure of the mud cake. Using the optimum MNPs, the CT number (CTN) shows a high CTN and density material, and uniform compacted thin mud cake structure (0.75 mm). The produced thin mud cake layer reveals the precipitation or exchange of elemental/mineral composition during fluid-solid interaction. These results confirm that MNPs could help in avoiding or reducing the formation damage, driving away drilling fluid from the formation, and improving borehole stability.

Cake formation and filtration characteristics of a cyclone-granular bed filter

Practical and improved ordinary differential models for compressible filter cake buildup involving small particle invasion and deposition inside the cake at static and dynamic conditions are developed by averaging particle and carrier fluid transport equations over the filter cake thickness. The models are simplified by considering that the panicles and carrier fluids involving many practical applications are incompressible. The results of radial and linear filtration cases are compared for constant rate and constant pressure drive filtrations. This thickness‐averaged ordinary differential model reproduces the predictions of the Tien et al. (1997) partial differential model rapidly with less computational effort. Parametric studies indicate that the particle screening efficiency of the filter is an important factor on the filter cake properties and filtration rate, and differences between linear and radial cake filtration performances are more pronounced, and the cake thickness and filtrate volume are smaller for constant pressure filtration than constant rate filtration. The equations of the thickness‐averaged linear and radial filter cake formation models are simple requiring less computational effort over the reported partial differential models for the analysis, design and optimization of the industrial cake filtration processes involving plate and drum filters.

A bentonite slurry mixed with seawater is prone to sedimentation, which will reduce the quality of the filter cake and lower the stability of the excavation surface in undersea tunnels. It is necessary to study the performance and influencing factors of the bentonite slurry mixed with seawater. This article simulates the process of undersea slurry shield tunnel construction, where the pressurized slurry penetrates into the sand layer and forms a filter cake when the shield stops pressurizing. We investigated the effects of bentonite, additives (CMC), fine sand, pressure, and formation permeability on the performance of the seawater slurry and filter cake. The sedimentation of mud caused by seawater interfered with the experiment, which manifested as the increase in bentonite, and delayed the formation of the filter cake. Fine sand with a particle size close to the average value of the formation can improve the speed and quality of filter cake formation. By conducting a sensitivity analysis on experimental data, the degree of influence of various factors on the formation rate, thickness, and porosity of the filter cake was determined. Fine sand and bentonite are the factors that have the greatest impact on the formation rate, thickness, and porosity of the filter cake.

An Experimental on Filtration and Clogging of Geotextile Filters around Drain Pipes in Fine Tailings

In horizontal open hole wells, the formation of filter cake while drilling the open hole section of the well is desirable. This filter cake serves the purpose of forming a semi-impervious layer around the reservoir drain-hole. This layer helps reduce losses considering the overbalance required for well control during drilling. It also serves as an additional structural support to keep the open hole stable when the drilling bottom hole assembly (BHA) is pulled out of hole and the screens and lower completions accessories are being run in hole. However, when thewell is put into production, the filter cake becomes a contributor to skin and poor reservoir productivity. It is therefore required to get rid of the filter cake after running the screens and the lower completion. Having procured and prepared the sand screens for deployment after drilling the open hole section, it is important that they are run to the bottom successfully with minimal damage and plugging. Usually, the open hole section of the horizontal well is drilled with specially formulated drill-in-fluids (DIF). Since this section is drilled in over balanced mode, the exerted pressure keeps the hole open so that the sand screen can be run successfully. The DIF replaces the drilling mud used to drill the earlier hole section(s) but in addition to providing well control via overbalance and transporting cuttings from the hole to surface, it also minimizes invasion damage to the reservoir pay zone. A commonly used weighing material when densities up to 11.5ppg are required for well control is calcium carbonate (CaCO3). When densities above 11.5ppg are required (for deeper, abnormally pressured reservoirs), it becomes necessary to weight up the mud with a heavier material, usually barite + CaCO3. During the drilling process, this overbalance pressure exerted on the reservoir forces the CaCO3 out of the DIF solution and it forms a semi-impervious filter cake on the sand face of the reservoir. This desirable filter cake helps minimize excessive fluid losses into the reservoir hence limiting invasion and damage. It also contributes to the structural integrity of the open hole, keeping it stable prior to running of the screens. Depending on the weighting material used in the drilling of the reservoir drain-hole, the micro-emulsion breaker (MEB) can be designed to break down the filter cake and any undisolvedparticulates can be mobilized and water-wetted and can be then flowed during production or injection. The challenge is that depending on the lower completion configuration, it may take some time to get the wash pipe and work string out of the lower completion and close the formation isolation device. In some cases, it is possible for the formation isolation device to fail. If the Micro-emulsion Blend (MEB) is quick acting, any of these scenarios can lead to uncontrollable losses and serious difficulties in continuing the completion operation. This elucidates the need for a delayed acting MEB treatment. Lab tests and analysis involving the exact DIF /filter cake and various compositions of the MEB at downhole conditions to arrive at the required delay in action. It is critical to ensure that the delayed action does not result in reduced efficacy of the treatment. Hence, the MEB is not diluted for slow action but rather it is engineered combinatorially with a retarder and downhole mild acid generating microemulsion chemistry that gradually generates the necessary mild acid that will slowly dissolve the bridging materials (eg. calcium carbonate) in the mud withtime and allows the full strength of the MEB to take effect after the stipulated delay period. This paper will focus on the lab analysis and iterations to arrive at an optimal MEB blend.

Cake formation and growth in cake filtration

Formation damage minimization and removal are important factors in enhancing field productivity and achieving targeted production rate. While formation damage is anticipated during drilling, an effective wellbore cleanup fluid becomes essential to remove mud damage and enhance well productivity. Filter cake layer created by drilling fluid can impair production considerably and should be removed prior to production using an effective treatment. Filter cake can be removed using a chemically designed filter cake removal treatment. The design of the treatment requires considerations of mud type and reservoir conditions. Also, it is important to consider the compatibility aspect of the treatment fluid with formation fluids and drill-in fluid base oil. Incompatible fluids result in precipitation which leads to further formation damage. In Saudi Arabia, many sandstone reservoirs are drilled with invert emulsion drill-in fluid to avoid the risk of clays swelling, which can lead to hole sloughing and formation damage. However, production impairment due to mud damage effect was observed in some horizontal wells drilled with invert emulsion drill-in fluid and completed with stand-alone screens as a result of improper mud managemet. After being put on production, their production rates have been reduced significantly with time. The decrease in production was related to screen blockage and mud cake impairment. A newly developed single-stage treatment was spotted and soaked to remove oil based mud cake and screen blockage. A significant sustained improvement in production was noticed and flowback solids indicated removal of screen blockage and mud cake. Lab studies were conducted at reservoir simulated conditions to evaluate the effectiveness of the treatment. The study covered testing the treatment filter cake removal efficiency, return permeability tests with reservoir core samples, and compatibility tests with formation fluids and drill-in fluid base oil. This paper will discuss in detail the laboratory work conducted and the successful field treatment performed.

Under the working condition of mud cake, the continuous action between the shield cutter head and the soil on the excavation surface can generate high temperature in the process of shield tunneling and excavation, which changes the characteristic of the adhesion between the soil and the cutter head to intensify the phenomenon of making a mud cake on cutter, finally leading to a vicious circle. To study the effect of temperature on the characteristics of the adhesion of the soil on the surface of the structure, the soil adhesive situation and adhesion force at different interface temperatures were tested through a self-made experiment device. According to the result, it was indicated that the moisture content has a significant effect on the adhesion force of the soil, and the adhesion force firstly increased and then decreased with the increased of the moisture content and reached the maximum value near the plastic limit moisture content. The adhesion force changes very gently when the interface temperature is low. When the temperature reached 50°C, the adhesion force continues to increase as the interface temperature continuously increases except for the soils with high moisture content; moreover, the interface temperature has a great influence on the content of soil adhered on the structure surface. As for the soil with moderate moisture content ( ω = 21.21 ~ 31 %), this content of the adhered soil increased exponentially with the increase of interface temperature; this content firstly decreased and then increased when the moisture content was high. When the soil was dry, there was almost no adhered soil on the surface and the interface temperature had no effect on the adhesive situation. By comparing and analyzing the adhesion state of the soil on the surface of the structure, the influence of temperature on the adhesion characteristics is mainly reflected on the variation of the soil moisture content within the influence range of the interface, the variation of the energy required for the destruction of the adhesion interface, and the change of the location of the weakest antistripping plane induced by both before. This research can better understand the law of formation and development of mud cakes and provided a new idea of solving the problem of mud cakes on the cutter head.

The filter cake layer, which is essential to reduce the fluid invasion to the drilled formation, has been reported to be composed mainly of the weighting materials. The complete removal of this layer is recommended to restore the rock permeability. Hematite is one material that is used to create the filter cake. However, the cleanup of hematite-based filter cake has not been studied thoroughly. The objective of the work presented in this paper is to investigate the removal of hematite water-based filter cake using hydrochloric acid. Numerous solubility tests were conducted to study the different factors affecting the treatment of hematite such as temperature and particle size. Based on these results, the treatment duration and acid concentration were selected. The high-pressure high-temperature filtration cell was used to build the filter cake and to soak it in the treatment solution at 100°C. Even though the solubility is increasing with acid concentration, however, no significant improvement was noticed above 12.5 vol.% of HCl. The particle size distribution has a considerable effect on the treatment, and the solubility was found to be 86 wt.% for particles finer than 25 microns and 47 wt.% for particle sizes above 75 microns, all in 10 vol.% HCl. The temperature has a significant influence as the solubility doubled when the temperature increased from 75°C to 100°C. The solubility rate decreased with time and reached the maximum in 16 hours which has been taken as treatment duration. Using 12.5 vol.%, 91 wt.% of the filter cake was successfully removed. In the literature comprehensive study on the cleanup of hematite-based filter cakes is lacking. This research provides new insights into hematite removal and address the different factors that impact the treatment efficiency.

Summary Many completion engineers use laboratory sand-retention testing as a tool to select a screen for standalone sand-control applications, some focusing on prepack and others on slurry testing. Those who use slurry tests for screen selection typically do so on the basis of the conventional wisdom that slurry testing is more challenging; thus, it represents the worst-case scenario for sand production. Furthermore, the general belief in the industry has been that metal-mesh screens with a "pore structure" are better for sand retention compared with wire-wrap screens (WWSs) of slot geometry, although they are more prone to "plugging." These are just a few of the many myths that exist in screen selection for standalone screen (SAS) applications. Recent papers on the modeling of sand retention by screens of various geometries, and supported by laboratory experiments, provided the tools for predicting sand production in both prepack and slurry conditions, as well as allowing for a systematic performance comparison of various screens by use of the entire particle-size distribution (PSD) of formation sands (Mondal et al. 2011 and 2012; Chanpura et al. 2012, 2013). In this paper, we discuss and challenge many myths in the screen selection for SAS applications and substantiate our findings with modeling and experimental data. The conditions under which a slurry or a prepack test would be more conservative are identified, highlighting the mechanisms of sand retention (size exclusion or bridging dominated). We demonstrate that the current thinking that prepack tests are always more conservative from a sand-production standpoint is incorrect. We also show that the concept that metal-mesh screens are always superior for sand retention than WWSs is incorrect, highlighting the factors that affect sand production through various screens [e.g., open flow area (OFA), wire thickness, and fraction of bridging-sized particles in the formation sand]. Finally, a methodology for screen selection in SAS applications is proposed.