Evaluation of Production Tests in Oil Wells Stimulated by Massive Acid Fracturing Offshore Qatar
Authors: McDonald Simon W. 
Published: December 2007

McDonald, Simon W.; SPE, Qatar General Petroleum Corp. (Offshore Operations)

This paper presents the evaluation of pressure-buildup data from production tests in wells that have been stimulated by massive acid fracturing. Fracture type curves are used in combination with conventional semilog analysis techniques. Fracture characteristics are calculated from a match of the early-time pressure data with the type curves, and reservoir characteristics are calculated from a conventional semilog plot of late-time data. Unexpectedly high formation permeabilities are evaluated, and fracture half-lengths are much shorter than design values.

Introduction

The Idd el Shargi field lies about 80 km offshore Qatar and consists of two culminations, called the North and South Domes, separated by a saddle. The Shuaiba reservoir is the shallowest oil-bearing reservoir in the field. It lies 4,500 to 5, 100 ft subsea and contains large volumes of oil trapped in predominantly low-permeability, chalky reservoir rock. Most of this oil is trapped in the North Dome. Offtake from the North Dome Shuaiba over the past 17 years has been primarily from three wells located near the crest of the structure where leaching has enhanced reservoir permeabilities and where well PI's are high. ranging from 10 to 60 B/D/psi. The depletion rate of the reservoir through these three wells historically has been very low. To date there has been no sustained off-take from the South Dome Shuaiba, and therefore both these reservoirs offer considerable development potential.

Most of the Shuaiba chalk is low-permeability reservoir rock, with nonimpaired (i.e., zero skin factor) well PI's calculated to be about 0.1 B/D/psi or less. One of the most significant factors about a potential secondary-recovery development scheme for this reservoir is the stimulation necessary to promote flow from or injection into the wells at acceptable rates. In recent years, massive acid fracture (WISPER-type-widely spaced etched ridges) stimulations have been carried out in selected pilot wells. Subsequent postfracturing production performances of these wells have proved the suitability of this process for stimulating the soft, low-permeability Shuaiba chalk.

This paper describes the analysis method employed by Qatar General Petroleum Corp. (Offshore Operations) to interpret the bottomhole pressure (BHP) buildup data from production tests carried out following acid fracturing the Shuaiba reservoir in the pilot wells. Of particular interest has been the calculation of the formation permeability thickness, kh, and fracture half-length, Xf, from these tests, as these will have a major impact on the design of a possible future secondary-recovery scheme.

Theory

Recently several papers have presented pressure drawdown data for various types of uniform influx, infinite conductivity, and finite-conductivity vertical fractures within a homogeneous isotropic formation. The data generally are presented in the form of log-log graphs (type curves) of the dimensionless pressure drop, PwD, vs. dimensionless time, tDxf, for the different fracture types.

JPT

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