Single-Stage Chemical Treatment for Oil-Based Mudcake Cleanup: Field Case and Laboratory Studies

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.

Evaluation of formation damage and filter cake removal with a new LPM drilling fluid additive

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.

This paper describes the dynamic and static filtration characteristics of four different drilling fluids under downhole conditions. Filtration rates were measured over two-, four- or five-day periods in a simulated sandstone formation. The drilling fluids studied had a wide range of API fluid loss and rheological characteristics. The API fluid-loss for all drilling fluids tested were generally higher than the filtrate loss observed in the sandstone formation. The largest static and dynamic filtration rates were seen in drilling fluids having uncontrolled fluid-loss properties. A large spurt loss volume, varying in duration from 30 seconds to five minutes occurred upon initial contact of the drilling fluid with the formation. A steady state filtration rate was quickly reached following the initial spurt loss. Dynamic filtration rates and the percentage of circulatable hole varied as a function of the drilling fluid flow rate in the annulus. Increased annular flow rates produced higher dynamic filtration rates. The dynamic filtration rates decreased prior to day four of the tests, afterwhich no significant changes were observed. Removal of the external wellbore filter cake using mechanical scrapers did not significantly change the dynamic filtration rate. The results presented can be used to estimate the effect of flow rates, fluid loss, and rheology of the drilling fluid on the amount of drilling fluid filtrate lost to the formation and on the percentage of the annulus that is circulating.

Appropriate drilling fluid additives which can build effective filter cakes are key components to minimize formation damage. Today there exists no effective and quantitative technique which can assess the formation damage minimization potential of any novel drilling fluid. This study provides the methodology for such a technique, by integrating HP/HT fluid loss data, derived from standard API HP/HT filter press, with filter cake quality and filtrate penetration depth, derived by Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) techniques. Novel nano-based drilling fluidwas made of de-ionized water, 7 wt% commercial Na-bentonite, and addition of custom made iron oxide (CM Fe3O4) nanoparticles (NP) at 0.5 wt%. Its excellent filtration characteristics were determined at elevated pressures and temperatures using a HP/HT filter press (500 psi/350°F) with ceramic discs, mimicking the formation, as filter media of k=775 mD. NMR T2 relaxation time measurements provided the pore size distribution of the ceramic discs, and a newly developed MRI technique by our team allowed direct visualization on the core-ceramic disc of the produced filter cake and filtrate penetration depth, providing insights on filtration characteristics. Addition of 0.5 wt% Fe3O4 NP resulted in a decrease of fluid loss by 47% compared to that obtained from the base fluid (BF) without the nanoparticles. The excellent filtration characteristics were attributed to the synergistic interaction of NP with bentonite platelets producing a strong microstructure network leading to rigid and very low permeability filter cakes. Spin-spin relaxation time (T2) NMR measurements revealed different pore size distribution of the nano-enhanced drilling fluid showing its tendency to occupy the smallest pores of the filter medium (ceramic disc). MRI T2 measurements revealed the saturation profiles of the drilling fluids inside the ceramic discs, in 3-D space. This allowed the visual representation of the penetration depth of the filtrate produced in the ceramic discs of the different tested drilling fluids. Our integrated characterization approach, gives us tools to analyze inter-particle interactions in the filter cakes at HP/HT conditions in order to explain the significant improvement in fluid loss performance of novel drilling fluids. The information gained in this study is considered to be extremely significant for deriving an effective methodology for assessing formation damage potential of any newly developed drilling fluid which can lead to more efficient and cost-effective drilling activities.

EZFLOW weak gel drilling fluid, a drilling fluid system with distinctive internal architecture, has been extensively implemented in horizontal well drilling operations at the Western South China Sea oilfields. Its unique internal structure causes specific functional mechanisms. The rheological mechanism was investigated through microstructural characterization, revealing that the microstructure comprises a reversible network structure with sol particles either encapsulated within the network or embedded at nodal points. This distinctive spatial network configuration endows the system with exceptional rheological properties. The plugging mechanism was elucidated via pre- and post-PPA test characterization of sand disc surface morphology. Experimental results demonstrate that the rheology modifier EZVIS forms deformable aggregates and films through intermolecular or intramolecular association in aqueous solutions, effectively plugging micro-nano pores/throats and microfractures to inhibit drilling fluid filtrate invasion. Concurrently, the rigid plugging material EZCARB establishes physical barriers at micro-nano pores/throats through bridging mechanisms. Notably, the dense filter cake formed by EZFLOW weak gel drilling fluid exhibits poor flowback characteristics, potentially inducing reservoir damage. Based on mechanistic analyses of rheological behavior, plugging performance, and filter cake composition, a filter cake removal fluid formulation was developed through: (1) creation of retarded acid HWCP to degrade polymer EZVIS and dissolve temporary plugging agent EZCARB; (2) development of corrosion inhibitor HWCI to mitigate corrosion rates. Laboratory evaluations demonstrated effective filter cake elimination and reservoir protection capabilities. Post-treatment analysis of EZFLOW-contaminated reservoir cores showed complete filter cake removal at core end faces with permeability recovery values exceeding 95%, indicating superior filter cake dissolution capacity and reservoir protection performance that significantly reduces formation damage.

The properties of the filter cake through maximum reservoir contact (MRC) and extended reach (ER) wells are not constant due to the presence of the drilled-cutting. The formation cuttings contamination in the drilling fluid during the drilling operation influences the drilling fluid properties as well as the filter cake properties. This study focuses on assessing the effect of the sandstone formation drilled-cuttings on the filter cake sealing properties and the internal solid invasion depth in horizontal section. Three different drilling fluid samples were collected at different points of a horizontal section during drilling operation. The drilling fluid samples were collected at the flow line coming from the well after shale shaker. High pressure high temperature (HPHT) fluid loss test was conducted to form the filter cake on top of 0.25 inch ceramic disc (35 micron). The composition of filter cake was evaluated using SEM and XRD. CT scan technique was used to evaluate the internal solids invasion of the different drilling fluid samples. The results from this study showed that the internal invasion is directly affected by the changes of sand contents in the filter cake. The internal drilling fluid solids invasion depth reached 3-4 mm when the sand content was lower than 25 wt % of the filter cake solids. Increasing the sand content more than 30 wt % in the filter cake solids allowed the drilling fluid to invade up to more than 6 mm with high concentration due to the degradation in the cake permeability. The result of this work will be useful to investigate diversion process to remove the formation damage from maximum reservoir contact and extended reach wells in sandstone reservoirs.

A key aspect during drilling operations is formation damage of productive formations by drilling fluids, which is examined to ensure an effective well completion. Drilling fluid is the blood of the well and the success of a drilling operation is closely related with its efficiency. The fluids provide pressure control in the wellbore, transfer the drilled cuttings to surface and seal possible fractures, thus preventing fluid loss and formation damage. Filtrate invasion can cause wellbore problems reducing productivity significantly. Thus drilling fluid formation damage characteristics need to be examined and controlled appropriately by using the right additives. Maintaining the optimal concentrations of these additives is vital for well integrity. Rheological and fluid loss characteristics are key fluid properties that need to be optimized for the development of stable and “smart” water-based fluids. The objective of this research is to develop appropriate additives in order to reduce formation damage in drilling operations. We do this by examining the fluid loss characteristics for water-based muds, utilizing iron oxide nanoparticles as fluid loss additives. The nanoparticles benefit from their small size and it is anticipated to seal porous and/or fractured formations and thus are expected to provide a great potential for reduction of formation damage. We present API filtrate loss and filter cake characterization along with the changes in the rheological properties of drilling fluids containing various concentrations of nanoparticles. We have measured rheological properties with Couette type viscometer both at low and high temperatures. LPLT and HPHT API filter presses have been used for fluid loss measurements. Scanning Electron Microscope (SEM) pictures were used to analyze the nanoparticle size range and to reveal secrets of their good performance by providing deep insights for their microstructure, the interfacial phenomena and the interaction between bentonite particles and the nanoparticles. The examined nanoparticles have the potential not only to significantly reduce the fluid loss and develop a thin mudcake, but also to maintain optimal rheological properties thus providing effective pressure control. The required relatively low concentration in the drilling fluid provides a base for more efficient, environmental friendly and safer drilling practices.

Predicting formation damage in cased-hole and open-hole completion wells is of high importance. This is especially relevant when the damage is caused by reservoir drill-in fluids hence being well-bore induced. Cake filter removal has proven to be a good approach to estimate induced damage and to evaluate drill-in fluids’ performance. We present an experimental methodology to evaluate filter cake removal, which could be achieved during the well's initial production. An improved experimental setup, to the ones presented in literature, has been developed to enhance data quality. A twofold approach was used for setup design, and first, it can be integrated with devices used to evaluate the static/dynamic filter-cake. Second, it can be used to simulate more realistic cases (field related) by adjusting the experiment parameters. Hence, to replicate the expected drawdown pressure as well as the corresponding flow rate of the studied reservoir. Three key indicators directly related to filter-cake removal were used as evaluators in this work. Lift-off pressure, internal and external filter cakes removal efficiency. Three reservoir fluid systems were studied, two polymer-based and one potassium carbonate. Results show that pressure required to initiate the collapsing process of the filter cake is not significant. Polymer-based drilling fluids showed better performance in terms of external and internal filter cake cleaning efficiency comparing to potassium carbonate. Moreover, we observed that filtrate volume has no clear relation with the degree of residual damage.

Drilling fluids are complex mixtures with different base fluids along with a series of additives to deal with specific well requirements. Oil- based drilling mud (OBM) filtration is commonly seen during the drilling stages and its removal is very critical to prevent impact reservoir productivity. Nowadays, exploitation in unconventional reservoirs, the closeness between producing wells and new drilled ones can create communication at reservoir level, resulting in reduction of the productivity index of the impacted parent well due to injected fluid migration. Removal of this induced formation damage can be very inefficient due to the usage of conventional acid formulations or solvents and traditional surfactants preflushes that are barely able to break and remove the existent damages, without actively water wetting the formation. An approach has been developed to eliminate the use of solvent preflushes, using a novel multi-functional chemistry in conjunction with 15% HCl to effectively remove OBM residue and any additional organic and inorganic damages present in the producing well. This innovative approach breaks and removes OBM emulsion damage while improving relative permeability reverting the wettability of the rock from oil-wet to water wet. This work performed (I would change to: This work involved) a suite of laboratory screening tests to evaluate the removal of filter cake and formation damage with the multifunctional chemistry approach in wells drilled with OBM or impacted for well communication. These tests included OBM filter cake clean-up in a high pressure/high temperature cell, dispersibility and reverse of the wettability of the particles, as well as fluid compatibility. Field data from clean-ups application with the multifunctional approach in conjunction with produced fluids samples and results from the laboratory testing, were utilized to validate the multi-functional chemistry performance. A multipackage formulation has been developed, to be fully miscible in acid packages, maximizing the performance of the OBM residual damage removal, and scale dissolution, by effectively de-oiling and penetrating the organic coating layer build-up on calcium carbonate and iron sulfide scales (typical damage in unconventional producing wells) in shorter soaking periods. This novel formulation cleans spontaneously by diffusion, breaking and solubilizing the organic deposits and providing the following additional benefits: Remove filter cake and formation damage created by OBM drilling and well communication.Reverse wettability of remaining particles (Water-wets) solids particles, preventing the re-agglomeration for further damage.Improve formation oil mobility by reducing the capillary pressure in the formation and maximizing productivity of the well. The work in this paper studies the effect of this novel chemistry when it is used in acid jobs, and presents case history information on testing, application, and subsequent field results across the Permian Basin unconventional production. Case history results from field trial is presented in this paper, demonstrating the advantages of utilizing this novel stimulation multifunctional chemistry in OBM removal to restore the productivity in wells damaged by OBM filtration during well communication.

As high-pressure/high-temperature (HP/HT) drilling is inherently expensive, drilling fluids and technologies should be carefully selected to successfully handle the associated challenges. Over the past few years, nanoparticles (NPs), among other additives, have been investigated to address these challenges. The objective of this study is to investigate the potential of using ferric oxide NPs on the filter cake properties in downhle drilling environments. Having an efficient filter cake is an important property of the drilling fluid and can affect the success of drilling operations. This research focuses oncharacterizatingthe filter cakes produced by Ca-bentonite-based drilling fluid contains ferric oxide NPs at downhle conditions. A combination of computed-tomography (CT) scan and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) was used in filter cake characterization. The effect of NP concentration, fluid preparation method, and filtration condition were studied. A HP/HT filter press was used to perform the filtration process at conditions up to 350°F and 500 psi. Indiana limestone core disks were used to simulate the filter media. The experimental results showed that the ferric oxide NPs improve the filter cake and filtration properties of Ca-bentonite-based drilling fluid in the presence of polymer and other additives. Low NP concentration is preferred for obtaining a good cake quality with the best characteristics obtained at 0.3–0.5 wt% NPs. Furthermore, this drilling fluid can withstand conditions up to 500 psi and 350°F. Cake properties of 0.151 in. thickness, 6.9 ml filtrate volume, and 0.428 µd permeability was obtained at such conditions. The addition of NPs to the drilling fluid improved the filter cake properties under both static and dynamic filtration. SEM-EDS analysis confirmed the efficiency of using NPs to form a smoother/less porous filter cake morphology. Moreover, sonication for one hour and hydration for 16 hours are recommendedfor better preparation of these fluids. This research provides an experimental evaluation of using ferric oxide NPs with Ca-bentonite-based drilling fluid to produce high-quality filter cake at downhole conditions. The characteristics of the filter cake gererated confirmed the efficiency of such fluids.

High-density mud is typically considered for use in well kill during workover operations. Weighting agents such as barite (BaSO4) and (Mn3O4) are utilized to weight up drilling fluids during workover operations to provide the high-density requirement to balance the reservoir pressure with a safe margin and prevent unplanned and unwanted flow back of reservoir fluid into the wellbore in high pressure–high temperature (HPHT) naturally fractured carbonate gas reservoirs. However, these kill muds can result in formation damage and reduced well productivity post-workover causing the well to require stimulation treatment to restore its productivity. Treatment evaluation at the laboratory level was conducted to determine the effectiveness of two fluid recipes: acid- and chelating-based, to remediate formation damage caused by high-density barite (BaSO4) and manganese tetraoxide (Mn3O4) weighting materials-based workover fluids. A commercially available formation damage simulator was utilized to examine the impact of the workover and treatment fluids on reservoir core samples at downhole conditions. X-ray diffraction/X-ray fluorescence (XRD/XRF) analysis and a baseline computerized tomography (CT) scan of the reservoir core plugs were conducted before core flooding. Differential pressures along core samples were measured at controlled flow rates during nitrogen gas flooding before and after the workover (Mn3O4/BaSO4-based) and treatment fluids (acid- and chelating-based systems) application. Darcy's equation was used to calculate return permeability values and core plug CT scans post-floods were used to describe the productivity improvement mechanisms. Before treatment fluid selection for core-floods, the laboratory rock and fluid compatibility testing were conducted. The laboratory results obtained confirm both the acid- and chelating-based treatment fluids were able to improve the well productivity by fully restoring the core permeability with a negative skin by creating wormholes/pinholes throughout the carbonate core plugs and by dissolving/breaking down the filter cakes. This paper presents a systematic treatment solution formulation and laboratory-based testing of treatment fluid's effectiveness in restoring productivity of Mn3O4 and BaSO4-based workover fluids formation damaged wells.

Non-aqueous drilling fluids (NADF) have become widely used due to their ability to provide failure-free drilling in difficult conditions. During the drilling, the filter cake is being built on the wall of the borehole providing the integrity of the wellbore and preventing formation damage. However, filter cake can cause long-term bringing-in of a well and an increased value of the drawdown required for inflow stimulation. Hence, the filter cake has to be successfully removed in order to maximize the well's productivity. The goal of the research is to provide a comprehensive analysis of various formulations used for NADF filter cake removal. NADF is typically formulated using oil as a continuous phase, brine as a dispersed phase, emulsifiers, wetting agents, and gellants. Depending on the reservoir, the filter cake may have a complex structure due to the particles’ intermixing with the emulsion to create the internal and external layers. Destruction of this system is an intricate task, particularly in horizontal and multilateral wells. An effective breaker must be able to separate the mud into component phases and simultaneously dissolve the weighting agents. The design of an effective treatment requires accounting for the mud composition, formation type, downhole conditions, and type of a completion. The variety of available solutions as well as their advantages and limitations are described in this article. The results of the research demonstrate that state-of-the-art systems and technologies can be divided into several groups depending on the number of stages and the required alteration of the rock wettability. To dissolve the external filter cake built by the hydrophobic weighting materials the chelating reagents, acids or acid precursors are commonly utilized. An internal filter cake formed by the emulsion drops and emulsion blockage in the damaged zone can be removed by solvents, mutual solvents, de-emulsifiers or microemulsions. Modern eco-friendly technologies as well as multifunctional thermochemical fluids used for removing the barite filter cakes are also highlighted in the publication. It is shown that many factors affect the filter cake removal efficiency, including mud oil-water ratio, types of solids present and their concentration, salinity, and temperature. Furthermore, a preliminary assessment of breakers’ performance should be performed in a laboratory while considering the individual characteristics of each well. Although many formulations have been introduced to remove barite filter cake in NADF, the need for a low-cost, compatible, and efficient breaker still prevails. Based on the research findings, a new thoroughly selected composition of chelates, surfactants, and additional reagents as a breaker with high de-emulsifying properties, and barite-dissolving ability is expected to be developed to break down the filter cake after completion of drilling with NADF.

Horizontal and maximum reservoir contact (MRC) wells are intended to increase productivity and minimize water production due to water conning. Their complex geometry makes cleaning up of drilling fluids filter cake a difficult task. It is hard to distribute the acid uniformly over long horizontal sections. Poor acid distribution occurs during matrix acidizing. Coiled tubing cannot reach the total depth of the well because of some limitations such as large washouts, length of the reel and diameter of the coil, which make acidizing horizontal wells ineffective. Available chemical methods of removing filter cake like mineral acids, esters, oxidizers, chelating agents, and enzymes are limited at certain conditions.This paper introduces a new method for filter cake cleaning using a self-destructing water-based fluid to drill long horizontal and MRC wells, which results in higher productivity. This fluid is a water-based mud that is weighted with calcium carbonate and has both functions of drilling and completion fluids, can reach total depth of MRC wells. It has the ability to effectively stimulate the whole horizontal sections after drilling. This can result in significant cost savings by elimination of acidizing using coiled tubing or any other means and improvement in filter cake removal and thus enhances performance of the treated wells.

The majority of wells today are drilled and completed conventionally. That is, they are drilled with kill-weight fluids which may or may not be detrimental to the formation. Wells that are drilled underbalanced, with less-than kill-weight fluid, foam or air, are often exposed to damaging fluids during completion operations. Technology new to the industry or currently being developed will help eliminate these problems and allow for a true underbalanced operation. This technology includes two-stage drilling, downhole lubricators, downhole swab valves, and isolation equipment used in fracturing operations. This paper addresses these issues and the importance that they play in a successful underbalanced completion.