Fluid Sampling From Porous Rock to Determine Filtrate Contamination Profiles Around a Well Bore

Drilling fluid filtrate invades the pores of rock surrounding a well bore during drilling of the well, and contaminates the pore fluid originally within the rock pores. Models for the flow of contaminated pore fluid towards a sampling tool within the well bore are investigated, assuming that the filtrate has the same viscosity as the original pore fluid and that the wellbore radius is small compared to the depth of filtrate invasion. If the filtrate contamination in the fluid withdrawn from the rock is monitored as a function of the volume withdrawn, then it is shown that results can be inverted to give the radial distribution of filtrate around the well bore. A new generation of guarded sampling probes is then considered, and it is shown that the radial distribution of filtrate can be obtained by means of such a probe if the fraction of flow entering the central sampling region of the probe is small compared to that entering the concentric annular guard probe. The effects of dispersion, non-zero wellbore radius and anisotropic hydraulic permeability of the rock are also studied, and numerical simulations are used to give some indication of the effect of the ratio of the filtrate viscosity to that of the original pore fluid.     

Phase distribution and intrapore salt exchange during drilling mud invasion of an oil- and gas-bearing formation

As a result of drilling mud filtrate invasion of a formation saturated with oil, gas and natural water, the distribution of the immiscible phases and the electrophysical characteristics of the near-well zone change as compared with its initial state. Taking this change into account is necessary for successful interpretation of electrical well logging data. In this paper, on the basis of the equations of immiscible fluid flow through a porous medium and the system of transfer equations with account for instantaneous salt exchange between the filtrate and the natural water inside the pores, the regions of initial formation fluid saturations for which in the invasion zone the displacement fronts retain their relative position are determined.     

One- and two-phase flow during fluid sampling by a wireline tool

Wireline sampling tools withdraw a few litres of fluid from a permeable formation via a small sink probe pressed against the borehole wall. The aim is to recover, quickly and cheaply, a representative native fluid sample. Unfortunately, the formation in the near wellbore region is invaded by mud filtrate, and withdrawal of nonnative fluid initially is inevitable. It is therefore of interest to estimate the proportion of native fluid in the sample stream, as a function of time.Semi-analytical calculations of one- and two-phase sampling flows are presented, for the special case of constant total fluid mobility in the limits of very deep or very shallow invasion. Both the interaction of the initially cylindrically symmetric invasion profile with the spherically symmetric flow and the capillary shock-forming dynamics of two-phase flow are found to control the character of sample composition variation. The wide variety of sample stream composition histories is displayed.     

Fluid-solid coupling model for studying wellbore instability in drilling of gas hydrate bearing sediments

As the oil or gas exploration and development activities in deep and ultra- deep waters become more and more, encountering gas hydrate bearing sediments （HBS） is almost inevitable. The variation in temperature and pressure can destabilize gas hydrate in nearby formation around the borehole, which may reduce the strength of the formation and result in wellbore instability. A non-isothermal, transient, two-phase, and fluid-solid coupling mathematical model is proposed to simulate the complex stability performance of a wellbore drilled in HBS. In the model, the phase transition of hydrate dissociation, the heat exchange between drilling fluid and formation, the change of mechanical and petrophysical properties, the gas-water two-phase seepage, and its interaction with rock deformation are considered. A finite element simulator is developed, and the impact of drilling mud on wellbore instability in HBS is simulated. Results indicate that the re- duction in pressure and the increase in temperature of the drilling fluid can accelerate hydrate decomposition and lead to mechanical properties getting worse tremendously. The cohesion decreases by 25% when the hydrate totally dissociates in HBS. This easily causes the wellbore instability accordingly. In the first two hours after the formation is drilled, the regions of hydrate dissociation and wellbore instability extend quickly. Then, with the soaking time of drilling fluid increasing, the regions enlarge little. Choosing the low temperature drilling fluid and increasing the drilling mud pressure appropriately can benefit the wellbore stability of HBS. The established model turns out to be an efficient tool in numerical studies of the hydrate dissociation behavior and wellbore stability of HBS.     

STUDY ON STEADY SURGE PRESSURE FOR YIELD-PSEUDOPLASTIC FLUID IN A CONCENTRIC

ＷａｎｇＨａｉｇｅ（汪海阁）；ＬｉｕＸｉｓｈｅｎｇ（刘希圣）（ＲｅｃｅｉｖｅｄＪｕｌｙ８，１９９４；ＣｏｍｍｕｎｉｃａｔｅｄｂｙＣｈｉｅｎＷｅｉｚａｎｇ）Ａｂｓｔｒａｃｔ：Ｓｕｒｇｅｐｒｅｓｓｕｒｅｃａｕｓｅｄｂｙｐｉｐｅｍｏｖｉｎｇｔｈｒｏｕｇｈｔ...     

Study on steady surge pressure for Yield-Pseudoplastic fluid in a concentric annulus

Surge pressure caused by pipe moving through the wellbore full of drilling fluid is the main factor affecting wellbore stability, on which the additional mud density is designed. Thus it is desired to predict the surge pressure with high accuracy. While this is closely related with the selection of mud rheological models. Yield-Pseudoplastic model which has three parameters and is famous for its higher accuracy has been used to describe the mud rheological properties in recent years. Based on this model the paper presents a new theoretical model for calculating surge pressure caused by mud viscosity during tripping or casing in a concentric annulus of directional wells under steady laminar situation. For convenience, the paper plots the distribution of the coefficients of surge pressure for different conditions. An example is given. These results provide the basis for controlling pressure surges and tripping velocity.     

Effect of the drilling mud filtrate temperature on the resistivity of the stratum saturated by oil and gas

A mathematical model of the axisymmetric distribution of the phases in the zone of invasion of the water-based drilling mud into the productive stratum whose porous space can simultaneously contain three immiscible fluids (oil, gas, and natural water) is constructed; the model takes into account the high rate of heat transfer between the fluids and the rock matrix. It is shown that the resistivity of the invaded zone depends not only on saturation of the latter by the fluids and the concentration of salts in the water phase, but also on the drilling mud filtrate temperature. It is also shown that there is a jump in the function of stratum saturation by oil on the thermal front.     

Invasion Zones in Lateral Drilling

A mathematical model for mud filtrate invasion in lateral drilling is proposed. The main assumption is that the difference in density between the invading and formation fluids is insignificant. Gravitational asymmetry of the invasion front is determined, and it is established that with time one of its points becomes abnormal and the entire invasion zone loses convexity. The main reason for the asymmetry is the density difference. If a lighter drilling mud is injected, the front floats up if the mud is heavier, the front floats down. The abnormal point of the front appears below or above the borehole, depending on the drilling mud weight. In the case where the mud is lighter than the formation fluid, the point of the front directly under the center of the borehole has the special property that with time it is less advanced downward than the neighboring left and right points of the front if the advance is reckoned from the horizontal axis through the center of the borehole. This property is the most pronounced for a small pressure difference between the borehole and formation equal to a certain critical value: under such conditions, the indicated point of the front does not move at all. For large pressure differences, the frontal advance is nearly equal in all directions.     

ANALYSIS OF THE WELLBORE STABILITY BASED ON PLASTIC DAMAGE THEORY IN SOFT SHALE FORMATION

软泥页岩在地层高压、流体环境下独有的力学变形特性一直是井壁稳定性控制工程领域的研究热点。针对软泥页岩低强度、高塑性的特点,考虑孔隙渗流、塑性损伤引起的渗透率变化等因素,建立了渗流-- 应力耦合形式的井壁稳定性评价数值模型,研究了流体渗流、泥浆压力、水平应力差的影响。结果表明,对过平衡钻井,当泥浆压力超过地层孔隙压力10% 时,渗流-- 应力耦合效应导致井壁塑性损伤区扩大至少65%,需予以考虑。水平应力差的增大会促进井壁塑性损伤区向最小水平主应力方向集中,且损伤区深度增大。     

Solution of pressure diffusion in radially composite reservoirs

This paper presents a new general method for solving the pressure-diffusion equation in cylindrically radial composite reservoirs, where the rock and fluid properties may change radially as a function ofr. Composite systems, such as formations with wellbore filtrate invasion and reservoirs with peripheral water encroachment, can be encountered as a result of drilling, secondary oil recovery, and water influx.The new solution method utilizes the reflection and transmission concept of electromagnetics to solve fluid flow problems in three-dimensional cylindrically radial reservoirs, where heterogeneity is in only one direction. The Green's function for a point source in a three-dimensional radially composite system is developed by using the reflection and transmission method. The method as well as the point source solution are sufficiently general that they may be applied to similar fluid flow and well testing problems involving single-phase flow.     

Hydrodynamic analysis of the displacement conditions of formation fluids using an axisymmetric model

The axisymmetric problem of the displacement of formation fluids by a drilling mud filtrate with filter cake formation is considered. An analysis is made of the distribution and variation of the main parameters of the process: filtrate volume, filter cake thickness, oil saturation, and pressure. The positions of the water-saturation and salt-concentration fronts are determined. The results are compared with the geophysical logging data for straight-hole drilling. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 2, pp. 56–64, March–April, 2008.     

Electromagnetic Sounding of Oil and Gas Formations

A mathematical model is proposed that describes electrical conductivity variation in the near-well zone during drilling formations containing three immiscible phases: oil, gas, and a small amount of native salt water. It is assumed that borehole drilling is performed using a clay–water solution, the mass-exchange process between the moving mud filtrate and immovable native water is infinitely fast, and displacement of the gas phase occurs by piston flow. The redistribution of the immiscible phases is described by the conventional Buckley–Leverett equations. The electromagnetic response of the medium is interpreted using the earlier proposed method of probabilistic convolutions.     

Invasion zone modeling using water- and oil-based muds

The hydrodynamic processes of displacement of reservoir fluids in an oil-saturated reservoir in vertical well drilling are studied by numerical simulation accounting for the drilling conditions and the type of mud (water-clay or oil-based). The influence of the hydrodynamic characteristics of the invasion zone on the specific electrical resistance depending on the type of mud is studied. The results of numerical simulations for the interpretation of high-frequency electromagnetic logs were used to construct geoelectric and hydrodynamic models in the reservoirs with different fluid saturations.     

Effect of the wettability of the reservoir rock on the flow into a well

The effect of the wettability of the reservoir rock on the productivity of a well in a flooded reservoir is analyzed for immiscible fluid systems of the oil-water type by means of a computational experiment. The calculations take into account the presence of a drilling mud filtrate penetration zone in the neighborhood of the well.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 172–175, September–October, 1990.     

Modeling of Rock Permeability Damage and Repairing Dynamics Due to Invasion and Removal of Particulate from Drilling Fluids

The investigation discussed in this paper was motivated by the need for model which is able to simulate both permeability reduction of hydrocarbon formations due to the mud-component invasion during over-balance drilling, casing/cementing, workover operations, and dynamics of permeability repairing during well cleanup. The paper focuses on development and validation of model to describe internal mud cake (IMC) dynamics, placing special emphasis on dynamics of the IMC removing during well cleanup procedure. Set of laboratory experiments with clay slurry injection and subsequent brine water backflow in samples of Bentheimer sandstone is discussed. The specific of these experiments is that backflow was carried out with alternating rates (“multirate” backflow). It is shown that the conventional deep-bed filtration model is not able to reproduce the dynamics of multirate backflow. The stochastic model we suggest takes into account pore size distribution and describes the mobilization of trapped particles within individual groups of pores within a “pore ensemble.” We provide simulation results to show that the suggested model reasonably reproduces permeability dynamics during both clay slurry injection and brine water backflow stages.     

Flow instability and shear localization in a drilling mud

To have a better knowledge of problems occurring with drilling fluids in complex wells, we carried out a detailed rheological analysis of a typical drilling mud at low shear rates using both conventional rheometry and MRI velocimetry. We show the existence of a viscosity bifurcation effect: Below a critical stress value, the mud tends to completely stop flowing, whereas beyond this critical stress, it reaches an apparent shear rate larger than a finite (critical) value, and no stable flows can be obtained between this critical shear rate value and zero. These results are confirmed by MRI velocity profiles, which exhibit a slope break at the interface between the solid and the liquid phases inside the Couette geometry. Moreover, this viscosity bifurcation is a transient phenomenon, the progressive development of which can be observed by MRI. A further examination of MRI data shows that, in the transient regime, the shear rate does not vary monotonously in the rheometer gap and is particularly large along the outer (rough) cylinder, which might be at the origin of the development of a region of constant shear rate in the apparent flow curve.     

Method of Probabilistic Convolutions for Interpreting Data of Electromagnetic Logging

A new method is proposed for processing the data of electromagnetic logging in wells. The method is based on the hydrodynamic analysis of drilling mud filtrate penetration into the oil stratum, determination of the field of electrical resistances of the near-well zone with its subsequent averaging by convolutions with a kernel in the form of a logarithmically normal distribution of probe sensitivity.     

井内流动与传热的三维耦合数值模型

钻井过程中,井内的压力、流场及温度分布是钻井作业必需的重要参数。本文提出了一个描述井内流动和传热的三维数学模型,用SIMPLER方法获得了该数学模型的数值解。该数值模型考虑了温度、压力对钻井液的流变性、密度以及各种介质的热物理参数的影响。现场试验数据与数值模拟结果的对比表明：本文提出的数学模型及采用的数值计算方法是可行的。     

Existence Result for a Radionuclide Transport Model with Unbounded Viscosity

We consider a model describing compressible nuclear waste disposal contamination in porous media. The transport of brine, radionuclides and heat is described by a nonlinear coupled parabolic system. The viscosity of the fluid is unbounded and concentrations and temperature dependent. Using a fixed point approach, we prove existence of physically relevant weak solutions.