Mudcake Buildup Research Articles

Assessment of True Formation Resistivity and Water Saturation in Deeply Invaded Tight-Gas Sandstones Based on the Combined Numerical Simulation of Mud-Filtrate Invasion and Resistivity Logs

The process of mud-filtrate invasion involves immiscible fluid displacement and salt mixing between mud filtrate and formation fluids in porous and permeable rocks. Consequently, the post-invasion spatial distribution of fluids and electrolyte concentration around the borehole affects resistivity measurements with different depths of investigation (DOI). In the presence of deep mud-filtrate invasion, the assessment of water saturation in the uninvaded zone based on the deep resistivity log can be inaccurate. Deep and electrically conductive filtrate invasion coupled with shoulder-bed effects can artificially increase water saturation (Sw) estimations by 20 saturation units (s.u.) in the Barik reservoir, resulting in pessimistic estimates of hydrocarbon pore volume if no corrections are applied. The Barik sandstone reservoir, which is characterized by low porosity ( up to 14%), low-to-medium permeability (up to 40 md), and high residual gas saturation (40 to 50%), exhibits low storage capacity to admit the critical filtrate volume necessary for building an impermeable mudcake. Combined with multiple days of overbalanced exposure to saline water-based mud (WBM), mud-filtrate invasion results in deep and smooth radial transition zones where the uninvaded formation is far beyond the depth of investigation of laterolog tools. Deep resistivity values are, therefore, lower than the true formation resistivity. Additionally, numerical simulations of resistivity logs show that the resistivity reduction by conductive invasion is further aggravated by shoulder-bed effects when individual reservoir thickness falls below 2.5 m. This paper describes the implementation of a compositional fluid-flow simulator to numerically model WBM-filtrate invasion and mudcake buildup in vertical boreholes. The algorithm allows the simulation of physical dispersion and fluid displacement around the borehole in a multilayer model. Time-dependent radial profiles of Sw and salinity are combined with core-calibrated porosity and electrical properties to compute electrical resistivity via Archie’s formulation. Subsequently, numerically simulated logs are generated using vendor-specific forward model processing and compared against field measurements. This workflow was extensively tested in various reservoir intervals with a wide range of petrophysical rock types and drilling conditions. Results show that the deep laterolog exhibits low sensitivity to conductive filtrate invasion when reservoirs’ porosities are lower than 8%. Above that threshold value, invasion length is a nontrivial process involving multiple variables. Even though exposure time to openhole conditions is a key factor leading to deep invasion, certain reservoir characteristics can lead to deeper invasion at short exposure times and significantly increase uncorrected Sw estimates.

Potential drilling hazards for wells targeting the Cane Creek shale, Pennsylvanian Paradox Formation, Paradox fold and fault belt, southeastern Utah and southwestern Colorado

The Cane Creek shale of the Pennsylvanian Paradox Formation represents a major target for oil and gas in the Paradox fold and fault belt of the northern Paradox Basin of southeastern Utah and southwestern Colorado. Early exploration and development attempts resulted in blowouts due to unexpected gas-bearing intervals and casing collapses caused by salt flowage in the Paradox Formation. These problems represent some of the types of drilling hazards that could be expected when planning Cane Creek wells. Horizontal drilling first used in the early 1990s changed the Cane Creek shale play from one of mostly drilling failures to a more successful commercial play. Depending on the location, exploratory Cane Creek wells may penetrate a section that ranges in age from Cretaceous through Pennsylvanian. Drilling in the region often encounters a wide variety of lithologies (carbonates, shale, mudstone, sandstone, and evaporites) and associated potential hazards that may include: (1) swelling clays, (2) high porosity-permeability or fractured zones resulting in lost circulation or excessive mudcake buildup, (3) “kicks” due to the influx of reservoir fluid (oil, water, or gas) into the wellbore, (4) uranium-rich zones, (5) washouts, (6) hole deviation, sticking, and other well-integrity problems, (7) chert, and (8) overpressured intervals. In addition, natural carbon dioxide, which flows from the partially human-made Crystal Geyser near some Cane Creek wellsites, represents an unusual drilling hazard if encountered in the northernmost part of the fold and fault belt. Using the lessons learned from the recently completed research well, State 16-2 (renamed the State 16-2LN-CC, API No. 43-019-50089, after the horizontal leg was drilled), and other wells in the region, drilling engineers and operators can better plan for potential hazards when exploring for hydrocarbons in the Cane Creek shale or deeper targets (Mississippian Leadville Limestone and Devonian Elbert Formation) in the fairly remote, relatively sparsely explored Paradox fold and fault belt. The goal is to de-risk wells, lower expenses, and mitigate problems before they occur. The expected results are safer and more successful drilling of wells to the Cane Creek shale and deeper reservoirs ultimately leading to additional commercial hydrocarbon discoveries in the region.

Experimental Investigation and Modeling of Mudcake Buildup Inside Fractures and the Resulting Fracture Isolation

Summary The role of mudcake buildup in fracture sealing and wellbore strengthening has been observed and documented in recent literature. However, there is a gap in the literature when it comes to fully understanding the mechanism of mudcake buildup, whether inside the fracture or at its mouth. This study presents an extension to a previously developed model, along with experimental validation, for mudcake buildup at the wellbore wall to capture the basic physics of mudcake buildup inside fractures and the resulting fracture sealing.

3D poro-elasto-plastic modeling of breakouts in deviated wells

Borehole breakouts are stress-induced enlargements of the wellbore cross-section. Predicting wellbore breakouts is important for the success of any drilling operation; however, accurate prediction of breakouts in deviated wells can be a technically challenging task. Misalignment between wellbore trajectory and field principal stress orientation along with the strong coupling between fluid flow, stresses and plasticity are some of the factors that makes breakouts in deviates well more challenging. This paper discusses a 3D poro-elasto-plastic numerical modeling approach for breakouts in deviated wells. Dynamic mudcake buildup on the wellbore wall is also incorporated into the numerical model, enabling quantitative analysis of the mudcake effect on the borehole breakout. The influences of well trajectory, fluid seepage between wellbore and formation pores, plastic strain softening of the rock, and dynamic mudcake buildup on the breakout are highlighted and analyzed using numerical examples. The results indicate that the breakout shape of deviated wells is largely dependent on well inclination while its volume can be reduced by mitigating fluid seepage through mudcake buildup. The proposed finite-element modeling approach in this study confirms the need to consider the coupling among these factors when predicting wellbore breakouts in complex geological and drilling conditions.

Multiporosity/Multipermeability Inclined-Wellbore Solutions With Mudcake Effects

Summary This paper investigates the effects of mudcake and formation multiporosity/multipermeability on the evolution of a safe-drilling mudweight window. The analytical poroelastic solutions for an inclined wellbore drilled through a multiporosity/multipermeability porous medium were derived, taking into account the mudcake buildup on the wellbore wall. A multiporosity/multipermeability porous medium consists of an overlapping of N distinct porous-continuum networks, each of which has its own geomechanical and petrophysical properties. Wellbore collapse and fracturing are investigated by studying a dual-porosity naturally fractured weak sandstone. The first porosity is matrix porosity, and the second porosity is the fractures. In addition, a triple-porosity source shale with multiple scales of natural fractures was modeled. The first porosity corresponds to the shale matrix one, and two scales of fracture distribution account for the other two sets of porosity. The Drucker and Prager (1952) criterion is applied to analyze wellbore collapse and shear failure, whereas the tensile strengths of both formations are considered, conservatively, as negligible. The safe-drilling mud-weight window is calculated to illustrate the time-dependent effects of formation N-porosity/N-permeability nature and the wellbore-wall boundary conditions (with/without mudcake). On the one hand, natural fractures narrow the safe-drilling mud-weight window by degrading rock strength and facilitating hydraulic-pressure invasion. On the other hand, the mudcake leads to a wider safe-drilling mud-weight window by generating compressive effective radial stress on the wellbore wall and impeding hydraulic-pressure invasion. Mudcake thickness and mudcake permeability are found to have significant effects. The drilling-mud design to build a mudcake on the wellbore wall is essential when drilling through difficult naturally fractured formations.

A predictive filtration model considering mudcake compressibility and non-uniform properties’ profiles

Mudcake buildup has a direct impact on expensive drilling and production problems, such as wellbore instability, pipe sticking, formation damage, and impairment of wellbore logging interpretation. Therefore, a better understanding of filtration and mudcake buildup is very valuable. This paper presents a predictive filtration model that accounts for the mudcake compressibility and captures non-uniform filtercake properties profiles. The model solves for the effective stresses acting across the mudcake thickness, based on which mudcake properties profiles are calculated. The mudcake is modeled as a stack of thin sub-layers, with each having its own properties. The porosity and permeability relations with effective stress are determined using the Odometer test, which is analyzed based on Terzaghi's theory for 1D consolidation. A detailed derivation of the model's equations is presented. Also, a computationally efficient pseudo-code solving the model's equations is suggested. Finally, the derived model is validated using experimental filtration results.

Wellbore breakouts: Mohr-Coulomb plastic rock deformation, fluid seepage, and time-dependent mudcake buildup

Borehole breakout is a time-dependent failure process which includes breakout initiation, propagation, and stabilization. Plastic rock deformation, fluid seepage, and time-dependent mudcake buildup on the wellbore wall affect the near-wellbore stress state and, therefore, the breakout behavior. In this paper, a hydro-mechanical model was developed for breakout prediction taking into account these factors. Filtration tests were conducted to obtain time-dependent permeability and thickness of mudcake, and the experimental testing results were incorporated to embody the dynamic mudcake buildup process. A sensitivity analysis was performed to investigate the effects of horizontal stress anisotropy, drilling mud pressure, and time-dependent fluid flow and mudcake buildup on breakouts in a vertical borehole. The simulation results show increased possibility of breakouts with larger horizontal stress anisotropy. Additionally, fluid seepage between the wellbore and the surrounding formation makes the breakout a time-dependent process. For low mud pressure, the initial breakout shape immediately after drilling is very similar to the final breakout shape after reaching steady state seepage. However, for high mud pressure, the wellbore may experience significant breakout propagation after drilling, owing to considerable fluid seepage associated with the larger differential pressure between the wellbore and the formation. Time-dependent mudcake buildup on the wellbore wall can effectively reduce the likelihood of borehole breakout by acting as a low-permeability barrier that mitigates fluid seepage across the wellbore wall and reduces changes in formation pore pressure. Disregarding the mudcake or considering a perfectly impermeable mudcake can lead to overestimating or underestimating the risk of borehole breakout, respectively. The proposed model provides a useful approach to understand and assess borehole breakout for drilling design.

An easy-to-implement numerical method for quantifying time-dependent mudcake effects on near-wellbore stresses

The stress state around a wellbore is critical for wellbore stability evaluation in drilling engineering. This paper presents an easy-to-implement numerical method for quantifying the effects of time-dependent mudcake thickness buildup and permeability reduction on the evolutions of stress fields around a wellbore during drilling. A common difficulty in developing dynamic mudcake models in most commercial or in-house numerical codes is the manipulation of mudcake thickness over time to represent mudcake buildup during the simulation. To overcome this challenge, a one-parameter description of both thickness and permeability variations during mudcake buildup based on an equivalent mudcake filtration study is used to facilitate model development, without compromising accuracy of the solution. The presented numerical method is verified against existing analytical mudcake buildup solution. Near-wellbore stress variation with mudcake buildup is investigated by implementing the proposed method to the commercial finite-element code Abaqus.

Modeling of filtration and mudcake buildup: An experimental investigation

This paper presents an experimental investigation and modeling of filtration and mudcake buildup during wellbore drilling. Extensive studies have been performed on this topic over the past seven decades and their general findings are summarized. In particular, the filtration model proposed in Dewan and Chenevert (2001) is discussed along with recommended methods for determining the model's parameters. New static filtration data were obtained using permeable filter discs manufactured from sand-cement mixtures to generate controlled formation material. The experimental results, consisting of fluid loss vs. time, mudcake porosity, and thickness at the end of the test, were used to evaluate the validity of the proposed methods and investigate the model's ability to predict the actual mudcake properties. Using the experimental results, some limitations of this model were identified. Therefore, modifications to Dewan and Chenevert's model were suggested to allow it to better capture the measured data, leading to the development of a new Wider Windows Filtration Model. Last, the effects of differential pressure, solids content of the mud, and filtration control agent on filtration and mudcake properties are presented and discussed.

Effect of nanoparticle behaviour on mud cake buildup for directional and horizontal wells: mathematical modelling and experimental study

ABSTRACTThe present study examined the effect of nanoparticle size and geometry on filter cake buildup during dynamic filtration using experimental and modeling approaches. A dynamic filtration setup was proposed and designed to test cross-flow circulation against a synthetic core at a constant differential pressure. The proposed mathematical model considers the critical deposition boundary of the porous media for particles along with drag, lift, friction, buoyancy, permeate, and electrostatic forces. The comprehensive model response allowed investigation of the aggregation and shape of nanoparticles at different cross-flow inclinations. The drag and lift force coefficients and moment of inertia of the nanoparticles were determined using cluster structures of nanoparticles identified using dynamic light scattering (DLS) and fractal theory. The deposition and erosion intensity index of all solid particles were experimentally determined and used in the model. Electronic microscope images of the mud cake surface confirmed the proposed cluster structure of the aggregated particles. The mud cake thickness measured by dynamic filtration tests and the nanoparticle-bentonite weight ratio acquired by DLS showed good agreement with the mathematical model results. The proposed approach allows selection of the optimum shape and size of the nano-solid particles as additives to minimize mud cake buildup along the borehole.

The Effects of Filter-Cake Buildup and Time-Dependent Properties on the Stability of Inclined Wellbores

Summary The effects of filter-cake buildup and/or filter-cake-property variation with time on wellbore stability have been plaguing the industry. The increasing use of lost-circulation materials (LCMs) in recent years for wellbore strengthening in weak and/or depleted formations necessitates models that can predict these effects. However, the complexities of effective-stress and pore-pressure evolution around the borehole while drilling, coupled with the transient variation of mud-filtration properties, have delayed such modeling efforts. In this paper, the analytical solutions for the time-dependent effects of mudcake buildup and mudcake properties on the wellbore stresses and formation pore pressure, and thus the safe-drilling-mud-weight window, are derived. The transient effects of mudcake buildup and mudcake buildup coupled with its permeability reduction during filtration on the safe-drilling-mudweight window are illustrated through numerical examples. The results showed that the safe-mudweight windows were greatly affected by the buildup of filter cake and its permeability variation. For example, the analysis for filter-cake buildup with cake permeability of 10–2 md showed that the safe-mudweight window was widened by 0.5 g/cc after 2.5 hours post-excavation when compared to the case of a wellbore without mudcake. On the other hand, a lower mudcake permeability of 10–3 md widened the mudweight window by as much as 1 g/cc. Last but not least, the analyses revealed that even for mudcake permeability as low as 10–3 md, neglecting the permeable nature of the mudcake can result in overestimation of the safe-drilling-mudweight window.

Estimation of Permeability and Permeability Anisotropy From Straddle-Packer Formation-Tester Measurements Based on the Physics of Two-Phase Immiscible Flow and Invasion

Summary We describe the successful application of a new method to estimate permeability and permeability anisotropy from transient measurements of pressure acquired with a wireline straddle-packer formation tester. Unlike standard algorithms used for the interpretation of formation-tester measurements, the method developed in this paper incorporates the physics of two-phase immiscible flow as well as the processes of mudcake buildup and invasion. An efficient 2D (cylindrical coordinates) implicit-pressure explicit-saturation finite-difference algorithm is used to simulate both the process of invasion and the pressure measurements acquired with the straddle-packer formation tester. Initial conditions for the simulation of formation-tester measurements are determined by the spatial distributions of pressure and fluid saturation resulting from mud-filtrate invasion. Inversion is performed with a Levenberg-Marquardt nonlinear minimization algorithm. Sensitivity analyses are conducted to assess nonuniqueness and the impact of explicit assumptions made about fluid viscosity, capillary pressure, relative permeability, mudcake growth, and time of invasion on the estimated values of permeability and permeability anisotropy. Applications of the inversion method to noisy synthetic measurements include homogeneous, anisotropic, single and multilayer formations for cases of low- and high-permeability rocks. We also study the effect of unaccounted impermeable bed boundaries on inverted formation properties. For cases where a priori information can be sufficiently constrained, our inversion methodology provides reliable and accurate estimates of permeability and permeability anisotropy. In addition, we show that estimation errors of permeability inversion procedures that neglect the physics of two-phase immiscible fluid flow and mud-filtrate invasion can be as high as 100%.

Dynamic Modeling of Invasion Damage and Its Effect on Production in Horizontal Wells

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 95861, "Dynamic Modeling of Invasion Damage and Impact on Production in Horizontal Wells," by P.V. Suryanarayana, SPE, and Z. Wu, SPE, Blade Energy Partners; J. Ramalho, SPE, Shell Intl. E&P; and R. Himes, SPE, Stim Lab, prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October. A radially adaptive fine-grid micro-simulator was used to model invasive damage during overbalanced drilling. Near-wellbore pressure and saturation distributions obtained from this model were used as initial conditions for a sector-scale model to study the effects of damage on long-term production performance. Introduction Overbalanced drilling and workover operations often result in invasion of the near-wellbore region by filtrate and solids from the drilling and workover fluids. This invasion causes a reduction in productivity because of degradation of effective permeability. During drilling, mudcake buildup can reduce invasion depth. Mudcake buildup and effectiveness depend on mud formulation, formation characteristics, and overbalance. In horizontal wells, mudcake effectiveness is compromised because of repeated pipe movement against the mudcake, which results in mudcake removal and reformation. Dynamic Damage Modeling A key feature of the model is a fine-grid dynamic microsimulator used to model invasion and obtain a near-wellbore profile of water saturation and reservoir pressure. Saturation-dependent effects such as relative permeability are modeled by use of relative permeability curves based on core tests. The microsimulator has an investigation range of approximately 50 ft from the wellbore. Inputs include the usual reservoir and drilling parameters such as mud properties, overbalance, and drilling time vs. depth. Simulations were performed with and without mudcake to represent the extremes of the invasion boundary. Results from a specially designed core test include undamaged effective permeability, resistance to flow from the wellbore into the reservoir because of the mudcake, absolute permeability damage, and mudcake liftoff pressure. Main output of the invasion model is a near-wellbore saturation and reservoir-pressure profile within the invasion zone that is used with the far-field reservoir properties outside the invasion zone as initial conditions in a sector-scale dynamic simulator with near-wellbore fine grids. Sector dimensions are chosen sufficiently large that no-flow or constant-pressure conditions apply at its boundary. Production is initiated under assumed conditions of rate and pressure constraints, and decline curves are obtained over a desired period of interest (usually 2 to 3 years) for each case of interest. The resulting decline curves for different options can be used to evaluate the effects of invasion on production performance. Fine-Grid Simulator In this work, the near-wellbore fluid-saturation distribution is solved from the two-phase immiscible flow equations as applied to a single-well model. A 2D axisymmetric cylindrical coordinate system is used. Permeability and porosity are assumed to be azimuthally symmetrical along the axis of the horizontal well, and streamlines also are uniformly and symmetrically distributed along the axis. Because the finite-difference method is used to solve the flow equation, extremely fine gridblocks are required to maintain uniform accuracy. A geometric progression scheme is used to generate radially adaptive gridblocks. Finer grid-blocks provide a seamless mechanism to capture the effects of permeability degradation near the wellbore. Typical gridblock size for reservoir-scale models is much larger than the radius of drilling-induced near-wellbore damage. Because a very short time frame (drilling) is being simulated, extremely small timesteps are used to minimize discretization errors.

Assessment of Permeability from Well Logs Based on Core Calibration and Simulation of Mud-Filtrate Invasion

This paper describes the application of a new methodology to estimate the permeability of clastic rock formations based on numerical simulation of the physics of mud-filtrate invasion. The methodology assumes a key well with a complete suite of well logs and laboratory core measurements of porosity, permeability, capillary pressure, and relative permeability. For additional wells in the same field, absolute permeability is estimated by matching shallow resistivity logs with the electrical resistivity in the flushed zone yielded by the simulation of mud-filtrate invasion. We describe the application of this methodology to the estimation of permeability in three wells penetrating the same over-pressured tight gas sands of the East Texas Bossier Formation. Conventional core porosity and permeability measurements, along with capillary pressure, are used to determine rock types and flow units in the cored key well. Estimates of permeability based on a modified version of Winland's equation agree with the measured permeability of the available core samples. Simulations of mud-filtrate invasion account for the dynamic process of immiscible flow between mud filtrate and in-situ gas as well as for salt mixing between mud filtrate and connate water. Moreover, the simulations properly reproduce the effect of mudcake buildup along the borehole wall. Two-dimensional spatial distributions of water saturation and salt concentration obtained from the simulations are used to calculate spatial distributions of electrical resistivity. The latter are checked against the measured shallow, medium, and deep resistivity logs to calibrate time of invasion and Archie's saturation and cementation exponents. Based on the analysis in the key well (Well 1), petrophysical assessment of flow units is performed in two nearby wells (Wells 2 and 3) within the same gas field. Initial Winland permeability values are progressively adjusted until the calculated spatial distributions of electrical resistivity agree with the shallow array-induction resistivity readings. We find that for Well 2, the estimated permeability is equal to the initial guess, whereas for Well 3 the estimated permeability is approximately 50-80% higher than the initial guess.

Numerical Simulation of Mud-Filtrate Invasion in Deviated Wells

Summary Understanding the spatial distribution of fluids in the near-borehole region caused by mud-filtrate invasion is necessary for the accurate petrophysical interpretation of wireline measurements acquired in deviated wells. This paper provides a sorely needed petrophysical and fluid-flow template that can be used to integrate several wireline measurements into a unique model of petrophysical parameters in the near-borehole region of deviated wells. We simulate numerically the physics of mud-filtrate invasion in vertical, horizontal, and highly deviated overbalanced wells. The numerical algorithm is adapted from a general 3D multiphase-fluid-flow simulator that is widely used in large-scale reservoir applications. Checks of numerical consistency and accuracy are performed against one commercial reservoir simulator. Emphasis is placed on describing the influence of mudcake buildup on the mud-filtration process. We approach the latter problem by introducing an effective-flow-rate function that describes the evolution in time of the rate of invasion of mud filtrate into rock formations. Parametric representations of the flow-rate function are derived on the basis of previously published laboratory experiments of mud circulation. A sensitivity analysis quantifies the influence of several geometrical and petrophysical parameters on the spatial distribution of mud-filtrate invasion away from the borehole wall. These parameters include relative permeability, capillary pressure, permeability anisotropy, dipping layers, and degree of hydraulic communication between adjacent layers. Our simulations reveal the character of invasion profiles in complex geometrical environments taking place under realistic petrophysical conditions. We show that standard pistonlike descriptions of mud-filtrate invasion, commonly used in well-log interpretation, can lead to inaccurate interpretations of wireline measurements. An example is presented of the use of our simulation technique by calculating the sensitivity of borehole electromagnetic induction measurements to specific conditions of mud-filtrate invasion in a vertical well.