Effects Of Mudcake Research Articles

An inversion method for calculating formation and borehole parameters in X-ray lithology density logging

The use of X-ray sources in place of the 137Cs sources used in traditional lithology density logging methods has become a new trend in the development of nuclear logging techniques. How to eliminate the effects of drilling fluids or mudcake in the measurement process is a key question that determines the accuracy of measurement. In order to reduce the effects of mudcake and improve the accuracy of measurement of formation parameters, this paper presents an inversion method that can accurately calculate formation and borehole parameters and is suitable for X-ray lithology density logging. The general process of this inversion method is described below. First, a response model for broad-beam attenuation during X-ray lithology density logging is derived. Subsequently, the responses of four detectors under various formation and borehole conditions are studied by means of Monte Carlo simulation, and the energy spectra measured by each detector are divided into four energy windows (ranges) depending on the correlation with formation parameters. Finally, accurate values of formation and borehole parameters are obtained through iterative inversion using the Levenberg-Marquardt (LM) algorithm. The results of this study show that compared with previously established analysis methods, the inversion method based on forward modeling can effectively improve the accuracy of measurement of formation density and lithology index during X-ray lithology density logging, reduce the influence of the borehole environment, and overcome the deficiencies of data processing techniques based on the spine and ribs plot.

Model Test and Numerical Simulation Study on the Effect of Mudcake and Length‐Diameter Ratio on the Bearing Capacity of Bored Cast‐in‐Place Piles

To study the effect of length‐diameter ratio and mudcake on the bearing capacity of bored cast‐in‐place piles, the indoor model test and finite element numerical simulation method were used to carry out the loading test of piles under different mudcake thicknesses (0 and 0.5 mm) and different length‐diameter ratios (10, 14, and 24). The variation law of pile top displacement, pile bearing capacity, pile axial force, and lateral friction resistance under the vertical load was analyzed. The results show that the larger the length‐diameter ratio of bored pile is, the more effectively the negative influence of mudcake around the pile on the settlement and end resistance of the pile is restrained, and the ultimate bearing capacity of the pile is improved. The research results have important scientific value and practical significance for understanding the influence of length‐diameter ratio on bearing capacity of bored cast‐in‐place piles.

Fully coupled thermal-hydro-mechanical model of pore pressure propagation around borehole with dynamic mudcake growth

Pore pressure is a significant parameter in drilling engineering. A better understanding of near-wellbore pore pressure propagation by taking into account the thermal-hydro-mechanical coupling process and dynamic mudcake growth is beneficial for wellbore stability analysis, formation testing while drilling and well logging while drilling. To reveal the pore pressure evolution mechanism, a new fully coupled thermal-hydro-mechanical model of pore pressure propagation around a borehole with dynamic mudcake growth is proposed. A comparison with the conventional approach suggests that neglecting the dynamic mudcake effect or the coupling effect can overestimate the pore pressure in the vicinity of borehole. The variation process of pore pressure can be divided into four stages, and the coupling effect weakens as time progresses. Compared with the Young's modulus of the rock, formation porosity, and temperature difference, the near-wellbore pore pressure is more sensitive to the overbalanced pressure, in situ stresses, formation permeability, and the Poisson's ratio of the rock. The near-wellbore pore pressure disturbance in formations with a low permeability and high in situ stress is more significant; therefore, to consider the effects of thermo-hydro-mechanical coupling and dynamic mudcake growth in deep and ultra-deep well drilling is required.

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.

Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

PREDICTION OF TUBING-CASING ANNULUS OVERFLOWS BASED ON THE LABORATORY STUDIES RESULTS

The results of studies of the mud cake effect on cement stone adhesion with reservoir rock are presented. A mechanism of overflows formation in the areas of the cement stone-reservoir rock contact has been determined. Some recommendations are proposed aimed at reducing the tubing-casing annulus overflows in a well.

Effects of a mudcake on Stoneley waves in a fluid-filled porous formation around a borehole

A mudcake formed on the borehole wall between a fluid-filled porous formation and the borehole fluid can affect Stoneley wave propagation used to estimate the formation permeability. The mudcake effect on the permeability dependence of radial oscillations of borehole fluid is investigated in a system with a source generating radial acoustic waves in the borehole and hydrophones (pressure sensors) that record the respective attenuating oscillations of the borehole fluid. The permeability of the porous formation is estimated from theoretical permeability dependence of the attenuation exponent, knowing the time dependence of the exponential attenuation of borehole fluid oscillations.

Dynamic Modeling of Invasion Damage and Impact on Production in Horizontal Wells

Summary We present a novel approach that combines dynamic reservoir simulations and special core tests to model the extent of invasive damage and its impact on flowback during production. A radially adaptive 3D microsimulator is used to estimate the extent and impact of filtrate invasion on near-wellbore saturation and reservoir pressure. Time-varying reservoir exposure is used to simulate the acts of drilling, tripping, completions, and workovers. Extremely fine, core-scale grids are used to capture saturation and pressure in the invasion zone. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Test results are interpreted to obtain an estimate of endpoint relative permeabilities, dynamic mudcake effect on filtrate loss, and impact of solids invasion on return permeability. The saturation and pressure profiles from this model are then used as initial conditions in a sector-scale simulator to model flowback effects. Absolute-permeability damage is modeled using the core-test results as an incremental and hyperbolically recovering effect during flowback simulations. A near-wellbore fine-grid overlay is used to capture the near-wellbore effects from the microsimulator results. Several sensitivities, including initial reservoir pressure, degree of overbalance and drawdown, heterogeneity, anisotropy, and mudcake effect, are examined. Equivalent skin factors that vary with time and depth are developed to enable comparison with full-field simulations. A horizontal-well example is used to illustrate the results of the study. Results illustrate the stark and often underappreciated effects of invasive damage on flowback and, therefore, on production performance. The methods described in this work can be used in reservoir-specific studies to quantify formation damage and aid in the selection of mud types, drilling techniques, and remediation methods required to improve performance. It is hoped that this work bridges the typically empirical damage-characterization methods and dynamic reservoir simulations.

Concept of Geometric Factor and Its Practical Application to Estimate Horizontal and Vertical Permeabilities

Summary In a probe-type formation test, because of the geometry of the wellbore and the sealing effect of mudcake, the flow pattern is not perfectly spherical. To account for the deviation from spherical flow, several geometric correction factors were proposed for different analysis techniques (Steward and Wittmann 1979; Wilkinson and Hammond 1990; Dussan and Sharma 1992; Goode and Thambynayagam 1992; Proett and Chin 1996). A geometric factor is used in formation rate analysis (FRA) (Kasap et al. 1999), a technique used in analyzing a probe test to estimate formation pressure and permeability. Like other geometric correction factors, the geometric factor is a strong function of permeability anisotropy that is generally unknown before a test. When analyzing the test, we would logically assume an isotropic formation and use the corresponding isotropic geometric factor. Consequently, the FRA-estimated permeability does not represent the true spherical permeability. In contrast, the spherical permeability can be estimated from buildup analysis without prior knowledge of permeability anisotropy. Therefore, there is a discrepancy between the permeability estimates from the two analysis methods. In addition, if considered separately, neither FRA nor buildup analysis can decompose the estimated permeability into its horizontal and vertical components. This paper presents the derived numerical values of several geometric factors. Using these factors, we show that the discrepancy between the permeabilities estimated from FRA and from the conventional buildup analysis is attributable to the permeability-anisotropy effect. A correct geometric-factor value must be used to estimate permeability correctly. On the basis of the permeability-anisotropy effect, we present the procedures to estimate horizontal and vertical permeabilities by combining FRA permeability and buildup permeability or by history matching. These procedures are verified with a simulated probe test. Analysis of three actual tests is presented. Introduction A formation test is initiated when a probe from a formation tester is set and sealed against the formation. A measured volume of fluid is withdrawn from the formation through the probe. The test continues with a pressure buildup when fluid withdrawal is halted. Pressure in the tool is continuously monitored throughout the test. The flow pattern during the formation test using a probe is believed to be close to spherical flow. Because of the geometry of the wellbore and the sealing effect of mudcake, however, such spherical flow is imperfect. To account for deviations from spherical flow, several correction factors have been proposed. These correction factors include flow shape factor (Steward and Wittmann 1979; Wilkinson and Hammond 1990; Dussan and Sharma 1992; Goode and Thambynayagam 1992), geometric shape factor (Proett and Chin 1996), and geometric factor (Kasap et al. 1999). Although they have different forms or names, their physical meaning is the same. We use a general term, geometric factor, to represent these correction factors in this paper. The physical meaning of these factors is that because the actual relationship between pressure and flow rate in a probe-type test cannot be described by a spherical-flow equation, these factors are added in the spherical-flow equation to correct for the actual flow relationship. These correction factors are a function of permeability anisotropy, among other parameters. Similarly, the concept of geometric factor is applied to estimate permeability with mini-permeameters (Goggin et al. 1988; Georgi and Jones 1992; Jones 1994, 1997).

Borehole dc resistivity response for a transitional invaded zone

A borehole direct‐current resistivity boundary value problem for normal and lateral elctrode configuratin is soved assuming axial symmetry. The borehole mud, a flushed zone, an invaded zone, and an unciontaminated zone are all assumed to be present. A linear transition in resistivity is assumed for the invaded zone. Frobenius extended power series and the method of separation of variables are used to solved the 1-D problem. Single-run borehole resistivity sounding and solution of the inverse problem are suggested fo estimatingthe resisitivity of the uncontaminated zone and the radius of invasion. Finite‐difference modeling is dione to estimate the effect of shoulder beds ion borehole sounding. Some of the computed 1-D and 2-D model apparent reisivity curves are compared with the existing scale model data. The analysis reveals that the mud cake effect is negligible for normal and lateral electrode array and the invasion zone thickness is feflected in the forward models. Apparent resistivity curves with and without a transitional invaded zone are well separated. Resistivity departure curves are well separated for fixed resistivity and variable resistivity invaded zone models. A normal electrode configuration can feel the presence of the shoulder bed in a 2-D model when the bed thickness is about 12 time the electrode separation. One‐dimensional ridge regression inversion the synthetic forward model data is presented to suggest an alternative approach for determining the resistivey of the uncontaminated zone ([Formula: see text]) and the radius of invasion [Formula: see text]. We conclude that (1) a single run borehole sounding with 10 or 12 data points from a normal or lateral log may be used, rather than 3 points from a dual laterolog [Formula: see text] tool, for better estimation of [Formula: see text], and (2) a borehole forward model should include a transitional invaded zone. Finally, an alternative approach for the estimation of the radius of invasion is proposed.

The Dual Spacing Formation Density Log

Abstract To overcome the problems of mudcake and hole irregularities, the new compensated formation density logging device employs two detectors spaced at different intervals from the source. The detector at the shorter spacing is particularly sensitive to the density of material immediately adjacent to the face of the pad. The contribution of this material, which includes mudcake and minor wall irregularities, affects the response of each detector to a different degree. The signals from both detectors are combined to give automatically a density correction which is added to the uncompensated density information from the detector at the longer spacing. Both the compensated density measurement and the amount of compensation are then recorded on the log. With the unwanted borehole effects removed, the measurement is recorded directly in terms of bulk density on a linear scale. Introduction Formation density is a very useful and diagnostic parameter for formation evaluation. When the matrix lithology is known, porosity is accurately and readily computed from density data. When the density measurements are used with other porosity-responsive measurements, such as Sonic and Neutron logs, both lithology and porosity may be defined with good accuracy. In the past, however, application of density logs was complicated by the need for corrections to obtain true formation densities from the log values. Mudcakes on permeable formations and roughness of the borehole walls have been particularly troublesome since, in each case, the device was prevented from direct contact with the formation. Attempts have been made to correct for such conditions by making a simultaneous caliper log, estimating pad standoff, and assuming knowledge of the composition of the intervening materials. While such corrections are frequently accurate, they are not always reliable because of uncertainties in the pad standoff and material composition. A further complication was the necessity for manual conversion of log counting rates to density. The newly introduced compensated formation density, logging device (FDC) automatically corrects for Mudcake and minor wall irregularities, and provides a log that is scaled directly in bulk density. The purpose of this paper is to discuss briefly the equipment, to explain the theoretical considerations involved in the compensation, and to present field examples of this new log. COMPENSATED FORMATION DENSITY EQUIPMENT The FDC tool incorporates two major advances over previous density logging devices: a second detector, spaced closer to the gamma ray source to provide increased sensitivity to material close to the pad; and the automatic derivation of a corrected formation density from the two counting rates. The arrangement of the source and detectors is shown in Fig. 1. The longer spacing, detector is at the same spacing from the source and gives the same response as the single detector in the previous uncompensated device. By appropriate combination of the outputs of the two detectors, the FDC tool is able to compensate in large measure for effects of mudcake and hole rugosity. This is achieved, as will be described later, without specific knowledge of either the thickness or the nature of material between the pad and formation. To obtain corrected formation density, the two count rates are automatically processed by an analog computer in the surface panel. JPT P. 1411ˆ