Interference Pressure Behavior in Multilayered Faulted Reservoirs
Authors: Sahni A. , Hatzignatiou D.G. , Ogbe D.O. 
Published: December 2007

Large scale reservoir heterogeneities such as fractures, faults or layers affect the performance and effectiveness of an IOR process significantly. Knowledge of the degree of communication between two wells separated by a fault is vital for designing improved oil recovery processes. This work presents an analysis of the interference pressure behavior in a multilayered faulted reservoir with sealing and non-sealing faults. A three-dimensional numerical model is used to generate transient pressure responses at both active and observation wells. A sensitivity analysis is conducted to examine the effects of the fault throw, formation transmissibility and storativity on the interference pressure behavior. Dimensionless pressure type curves are developed and studied for the system under consideration; based on these type curves a methodology is developed which can be applied to obtain estimates of the fault and formation transmissibility as well as the formation storativity. Reservoir parameters estimated using the well test method developed in this study can be used in the simulation of IOR processes and in optimizing field development plans with respect to the optimum number and location of wells to maximize recovery from an oil reservoir.

Introduction

Reservoir heterogeneities arising from deposition, folding and faulting, post-depositional changes in reservoir lithology, and changes in fluid type or properties may affect the wellbore pressure response significantly. Heterogeneities can be small-scale, such as fractures, vugs or solution cavities in carbonate reservoirs or large-scale, such as physical barriers, faults, thickness changes, lithology changes, faults and fluid-fluid contacts. Many tectonic movements do not result in a complete throw and often permeable beds, perhaps corresponding to different facies, may at-least be partially adjacent across the fault plane. Such a barrier is known as a non-sealing or partially communicating fault since it allows some degree of lateral fluid flow across it. In some cases, two sections of a permeable formation may be displaced to such an extent that both faces are adjacent to impermeable rock. This barrier is known as a sealing fault since it blocks fluid and pressure communication with other regions of the reservoir; Fig. 1 illustrates some important aspects of faulting in multilayered reservoirs. The two layers are separated by an impermeable shale barrier which prevents any fluid or pressure communication between them. However, tectonic movement may displace the layers in such a way that they might come in contact across the fault plane. Tectonic movement will also cause an alteration in permeability around the fault plane which would affect the pressure behavior.

It is pertinent to note that fluid flow can occur across the fault in the horizontal direction and also along the fault plane (F-F), in the vertical direction (Fig. 1). Fluid flow can also occur within the fault along the fault plane, depending on the permeability of the fault. The fault plane usually has a different permeability than the adjacent formation as a result of tectonic and mechanical forces such as bed deformation, grain crushing, clay smearing and possible geochemical alterations.

Although seismic data may provide information regarding the existence of faults, it is extremely difficult to ascertain from geological and geophysical data whether a fault is sealing or nonsealing. Well test analysis may be used to determine the existence and degree of communication across a fault. The transmissibility or fluid transfer capability of faults is a key parameter in determining the number as well as location of wells for maximizing drainage and oil recovery from a faulted reservoir. Hence, it is essential to determine accurately the fluid transfer capability of a fault for the development planning of oil and gas fields.

Reservoirs containing a linear sealing fault case have been discussed and investigated extensively in the literature. Most of the previous research has been directed towards single or multiple linear sealing boundaries in single layered reservoirs.

In one such study, Stewart et al. used a reservoir simulator to obtain observation well pressure response on the far side of a partially communicating fault from a constant rate active well.

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