Utilizing Welltests in Determining the Drive Mechanism and Threshold Pressures Between Three Communicating Reservoirs
Authors: Zubari H.K. , AL-Awainati M.Y. 
Published: December 2007

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The implementation of a welltesting program of Build Up and Draw Down tests conducted on wells producing with high GOR from a thin tight oil zone has revealed a unique and consistent communication pattern between three communicating reservoirs. Analysis of the tests showed conflicting response; in Draw-Downs, for instance, Constant Pressure Boundaries (CPB) were detected, whereas in Build Ups, barriers were encountered for the same well. The tests revealed the presence of a threshold pressure which controls communication between the reservoirs and hence the high gas production rates from the tight formation. The welltesting program shed light on the drive mechanism of the tight formation and explained the conflict between the production and pressure data experienced throughout the history of the reservoir. Based on the findings which were further supported by a simulation model, interference tests, and statistical distribution of production methods in the area of interest, a secondary recovery method was designed to provide a solution.

This paper highlights the effective role of welltesting in understanding the drive mechanism of a complicated reservoir and utilizing the information in improving its recovery.

Introduction

Being under gas injection for over 50 years, the Mauddud reservoir developed a thick gas cap in the central area of the field. With the presence of many faults in the area, the Mauddud gas cap caused early gas breakthrough in the overlaying reservoirs, Aab and Ac, causing wells to produce dry gas and the reservoirs to have very poor recoveries. Despite the confirmed communication between the reservoirs, the static pressure of the overlaying reservoirs, Aab and Ac, never reached that of Mauddud gas cap even after months of shut in. Therefore, in order to understand the communication patterns and develop a plan to optimize oil production from both reservoirs, a welltesting program was implemented on Aab wells in the area. The tests, mainly Build Ups and Draw Downs, revealed a unique and consistent communication pattern through threshold pressures across fault planes. The tests also revealed the role of the intermediate zone, Ac, in providing a conduit to gas flow from Mauddud to Aab.

The process was confirmed by a simulation model, interference test and a study on the statistical distribution of naturally flowing vs. artificially lifted wells in the area of interest. The simulation study indicated that if Aab wells are left shut in for several years, the pressures between Aab and Mauddud would eventually equalize indicating the threshold pressures to be acting only initially.

Based on the above, it was possible to quantify the short-term threshold pressure between the three reservoirs. This further allowed the development of an engineered recovery scheme whereby, waterflooding Ac is expected to increase the pressure of Aab and Ac to be always below the minimum threshold pressure, thus; eliminating or minimizing Mauddud's gas cap encroachment into both reservoirs.

Background

Geology. As shown in Fig. 1, the three layered reservoirs under study; from bottom up: Mauddud, Ac and Aab, belong to the Wasia group of the Middle Cretaceous age. The Mauddud limestone thickness averages 110' and has a permeability in the range of 100–300 md. It has a gas cap thickness in the range of 20'–80' in the Area of Interest (AOI) and is overlain by the Wara formation which varies from 60 to 95' in thickness. Its erratic sandstone facies, Ac, distribution has a good permeability of 200–300 md and varies from 0 to 60' in thickness. The limits of this sand body forms the AOI of this study. P. 315^