An Approach To Predict Tarmat Breakdown in Minagish Reservoir in Kuwait
Authors: Osman Mohammed El-Sayed 
Published: December 2007

Osman, Mohammed El-Sayed, SPE, Kuwait Oil Co. (KSC)

The Minagish oolite reservoir, Minagish field, Kuwait, is characterized by the presence of tarmat at the oil/water contact (OWC), so a waterflooding project was planned for the reservoir. This paper discusses the possibility of tarmat breakdown by injection of water below it. The differential pressure at the tarmat resulted mainly from water injection, and this differential pressure, a result of oil production, was negligible. This paper presents a technique to predict tarmat breakdown time, response time at the nearest producer or observation well, and the time when water injection should be switched from below the tarmat to above it. Also, this technique can be used to predict the differential pressure at tarmat anywhere in the predict the differential pressure at tarmat anywhere in the reservoir.

Introduction

Asphaltic deposits and tarmats have been recognized for many years. Tarmats normally occur near the base oil accumulation or near the surface as oil seeps. Several recent geochemical studies indicate that tarmats formed as a result of one or more of the following mehanisms.gravitational segregation that caused the hydrocarbon fractions to stratify with the lighter petroleum at the top of the reservoir and the heavier at the base.natural deasphalting where natural, buoyant gases from the source rock entered the pool and rose through the hydrocarbon column, lowering the solubility of the asphaltic fraction. which would consequently precipitate and fall to the base of the reservoir; andwater washing, the movement of undersaturated water that removed a portion of light hydrocarbons, leaving asphaltic fractions at the base of oil accumulation.

The moving water might have carried bacteria that selectively metabolized the lighter fractions of crude oil. This might have caused a tarmat to form.

One oil reservoir with tarmat is the Minagish oolite reservoir. It is located in the west central portion of Kuwait, as shown in Fig. 1. The presence of tarmat was indicated by samples, cores, and logs that were obtained from several wells drilled in the reservoir. The mobility of the tar was almost zero. It created a zone that isolated the reservoir from its aquifer. The aquifer was a good size. The reservoir, however, resembled a depleted-drive type. Thus, a gas-injection project was begun in 1967 to maintain reservoir pressure. Since 1971, the' reservoir pressure has remained about 4,200 psig [29 MPa] without pressure has remained about 4,200 psig [29 MPa] without any decline.

Pressure surveys of different wells indicated pressure support at different locations in the reservoir. Moreover, we interpreted the rapid pressure decline (450 psi [3.1 MPa]) at initial production followed by a steady pressure buildup after field shut-in as a result of breakdown in the tarmat. Both conventional and simulation studies were performed on the reservoir. It was impossible to explain the performed on the reservoir. It was impossible to explain the reservoir's behavior without considering tarmat breakdown in certain locations in the reservoir.

A plan to waterflood the reservoir was enacted. The main objectives of injecting water below the tarmat were to recover some of the heavy oil (tarmat) and to increase the communication between the aquifer and the reservoir, and, consequently, to use its energy in producing the reservoir.

This paper discusses tarmat breakdown after water is injected and the duration of that injection.

Model Description

For the purpose of this study, the Minagish oolite reservoir was divided into five layers on the basis of permeability. Tarmat was present at the OWC. It acted as a permeability. Tarmat was present at the OWC. It acted as a complete barrier between the aquifer and the reservoir. Its thickness varied from 30 to 115 ft [9.14 to 35.1 m]. We planned to waterflood the reservoir in a peripheral flood pattern. To predict the full-scale flooding performance, two waterflooding pilots were chosen-one in the performance, two waterflooding pilots were chosen-one in the north and the other in the south. The northern pilot injector was located in the reservoir where the tarmat was relatively thick, and the southern one was located where the tarmat was relatively thin. The results of the two pilots would indicate whether to inject water above and/or below the tarmat throughout the rest of the reservoir. Furthermore, the decision had to be economically feasible. Data from the northern Pilot Injector MN-26 were used in this study. Fig. 2 presents a structural cross section of the MN-26 injector and the tarmat. The average rock properties are indicated on the figure. Clearly, Layer 3 properties are indicated on the figure. Clearly, Layer 3 is the thickest and most permeable layer. Data from Producer MNA (shown in Table 1) were used to study Producer MNA (shown in Table 1) were used to study the effect of oil production on the differential pressure at the tarmat.

The following assumptions were used in this study.The reservoir and its aquifer behave as an infinite-acting reservoir. This is necessary to validate the solution of the diffusivity equation presented later.The tarmat is a complete barrier between the reservoir and its aquifer until it breaks down.

JPT

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