Pressure Maintenance by Formation Water Dumping for the Ratawi Limestone Oil Reservoir, Offshore Khafji
Authors: Fujita Kazuo 
Published: December 2007

An analysis technique is presented that examines early time pressure data. Such a technique is useful when the middle time region of data, customarily required for the Homer analysis, is distorted by reservoir complexities such as gas cap, water intrusion, interbedded shales, natural fractures, etc.

Introduction

Hydrocarbon reservoirs rarely conform to a simplified model with a standard behavior. Real reservoirs are inherently complex, with their properties still undergoing changes as our intervention advances. Producing fields are seldom homogeneous. Production of one formation zone causes induced flow from adjacent formations with fluids crossflowing, through bounding semipervious beds. Sustained production results in expansion of existing, or creation of new, gas cap areas. Water encroachment from underlying, aquifers further affects the producing fields. Reservoir characteristics and fluid properties become essentially time-variant. Under these circumstances. conventional well testing analysis based on some middle time region (MTR) of the pressure data curve, the region most vulnerable to the reservoir heterogeneity, has doubtful applicability. If modification of the MTR is not recognized, the analysis becomes subjective and often inconsistent with the actual well behavior. In addition, interpretation of such an analysis is nonunique since many types of heterogeneities give similar pressure responses. The effect of some reservoir heterogeneities and the manner in which these heterogeneities modify the MTR of the pressure curve are discussed in the next section. Then follows the development and, finally, the application of the analysis based on examining early time pressure data, the data that are least, if at all, influenced by reservoir heterogeneities.

Effect of Reservoir Heterogeneities on Producing Well Pressure Behavior

In this section the effects of formation heterogeneities such as interbedded shales or natural fractures that respond to the reservoir production with different time lags are discussed. The effect of a gas cap or water leg that offers pressure support to the oil-producing formation is included also. These heterogeneities distort the MTR buildup data on a Homer plot to give multiple straight lines or a curve with an ever-decreasing slope, thus invalidating the standard semilog analysis.

Shaly zones with a measurable permeability that are adjacent to a producing formation may influence the pressure behavior of the formation considerably. Serving as semi pervious beds, shaly zones offer a delayed response to production by allowing vertical crossflow that enhances the fluid flow to a producing well. The well pressure response at the MTR-i.e., at the tune when the effects of after flow and the well partial completion die out-is found to consist of not one but two straight-line segments (Fig. 1). The first segment represents a transitional region during which pressure is affected by crossflow from shaly sections. The second segment shows the pressure response after the time variant crossflow has stabilized. The slope of the second segment is twice that of the first. This second segment is the true MTR that is indicative of the true formation permeability and long-term reservoir productivity. Since the pressure segment preceding the MTR also is a straight line, it can be confused with the MTR, especially if a limited buildup time did not allow development of the true MTR of pressure data. If misinterpreted for true MTR and analyzed by the Homer technique. the transitional straight-line segment will yield a reservoir permeability twice the actual value.

The pressure behavior of a producing, formation will be even more complex if the confining semi pervious shales are in turn overlain by another permeable formation.

JPT

P. 1145^