Numerical Simulation of Mud-Filtrate Invasion in Deviated Wells
- Jianghui Wu (U. of Texas at Austin) | Carlos Torres-Verdin (U. of Texas at Austin) | Kamy Sepehrnoori (U. of Texas at Austin) | Mojdeh Delshad (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2004
- Document Type
- Journal Paper
- 143 - 154
- 2004. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.3.4 Scale, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.8 Formation Damage, 5.5 Reservoir Simulation, 4.1.5 Processing Equipment, 1.6 Drilling Operations, 5.6.1 Open hole/cased hole log analysis, 1.14 Casing and Cementing
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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. Empha- sis 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.
Mud-filtrate invasion takes place in permeable rock formations penetrated by a well that is hydraulically overbalanced by mud circulation. This condition is of interest in numerous oilfield applications including drilling engineering, reservoir simulation, reservoir stimulation, and well-log interpretation. Electrical, electromagnetic, acoustic, and nuclear logging instrument responses are all influenced by the spatial distribution of fluids in the vicinity of the borehole resulting from the invasion of mud filtrate.
The invasion of mud filtrate into permeable rock formations is responsible for the development of mudcake on the borehole wall (solids deposition), as well as for the displacement of existing in-situ fluids laterally away from the borehole.1-5 Standard procedures used for the interpretation of well-logging measurements often conceive of the invasion of mud filtrate as a radial sequence of pistonlike fluid-saturation fronts. There have been a handful of techniques put forth to simulate (numerically and in the laboratory) the physics of mud-filtrate invasion. Drilling variables such as mud density and chemistry, mud circulation pressure, and time of filtration may all significantly affect the spatial extent of mud-filtrate invasion. In-situ rock formation properties such as porosity, absolute permeability, relative permeability, pore pressure, shale chemistry, capillary pressure, and residual fluid saturations also play important roles in controlling both the dynamic formation of mudcake and the time evolution of the invasion process.
Simple 1D models of mud-filtrate invasion exist based on the assumptions of a vertical well and a horizontal and infinitely thick rock formation.2 These models have been derived by the enforcement of mass-balance conditions and at best assume a homogeneous and isotropic spatial distribution of porosity and permeability while neglecting capillary forces and relative permeability effects. To date, there are no available numerical algorithms capable of simulating the physics of mud-filtrate invasion in rock formations comprising multiple hydraulically connected beds, nor are there algorithms that can simulate the geometrical complexity associated with deviated or horizontal wells.
In this paper, we introduce a general numerical algorithm to simulate the physics of mud-filtrate invasion in vertical and deviated boreholes. This algorithm is adapted from an existing 3D multiphase-fluid-flow simulator widely used to replicate the behavior of large-scale hydrocarbon reservoirs. The simulator, developed by the U. of Texas at Austin, is commercially referred to as UTCHEM.6-9 A detailed description of UTCHEM's multiphase, multicomponent, and multichemical-species fluid-flow model is presented in the UTCHEM technical documentation.10
One of the salient technical problems often considered in mud-filtrate-invasion studies is the description of mudcake buildup. The development and thickening of mudcake on the borehole wall causes the permeability of mudcake to decrease monotonically. In turn, this causes the flow rate of mud filtrate to decrease toward a low steady-state value. Both chemical and fluid mechanical processes determine (a) the rate at which solids accumulate on the borehole wall and (b) the thickness and effective permeability of the mudcake. Complications may arise as a result of periodic retrieval of drillpipe for bit changes. This can cause scraping of mudcake, resulting in secondary mudcake buildup and, hence, secondary mud-filtrate invasion.
Recently, and on the basis of laboratory experiments of mud circulation, Dewan and Chenevert1 reported a methodology to predict the time evolution of mudcake buildup as well as the effective petrophysical properties of mudcake. Dewan and Chenevert's description is based entirely on six mud-filtrate parameters, all of which can be determined from a standard mud-filtrate test. As expected, Dewan and Chenevert's work predicts a monotonically decreasing rate of mud-filtrate invasion as a function of time. Moreover, Dewan and Chenevert emphasize the fact that the rate of mudcake thickening remains practically unaffected by the petrophysical properties of the invaded rock formation. Borrowing from these results, in our simulations of mud-filtrate invasion we have completely avoided the problem of modeling mudcake buildup. Instead, we have chosen to model the effect of mudcake buildup on the invasion profile by defining an equivalent time-dependent flow rate of mud-filtrate invasion.
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