Recent Field Results With New Bits
Authors: van Prooyen Jan 
Published: December 2007

Multiple pulse tests were conducted in Saudi Arabia to yield information about permeability anisotropies in the Ghawar oil field. Because of wide well spacing and high permeability, sensitive quartz pressure gauges were used to record pressure changes. Pressure data from observation wells were processed to filter out noise and to provide easily definable pulse responses. Calculated pulse test results then were analyzed for indications of directional permeability.

Introduction

To date, three Multiple production pulse tests have been conducted in the Ghawar oil fileld in Saudi Arabia in a continuing effort to improve the reservoir description used in reservoir simulation models of the area. Results from and procedures used in a 1977 test are presented by McIntosh et al.1 The results of the other two tests, which were conducted in 1979, are the subject of this paper.

The performance and accuracy of any reservoir simulation study are related intimately to the completeness of the reservoir description of the field being modeled. Pulse tests, interference tests, and single-well pressure transient tests all provide valuable information without reservoir properties. The three pulse tests conducted in Ghawar were run to investigate the area distribution of permeability and, in particular, to test for the presence of area permeability anisotropies in the productive formation. Major directional permeability can have a dramatic effect on fluid movement in the reservoir; therefore, the description of permeability trends is a key element in reservoir modeling. Results from the pulse tests are being integrated with results from cores, logs, and other well tests to improve overall description of the reservoir.

The previous pulse test conducted in 1977 proved the feasibility of pulse testing in the high-permeability, wide well-spacing environment of Ghawar. The two tests discussed in this paper were located to provide good areaI coverage over the central portion of Ghawar. One test was run near an area that previously had been identified by buildup testing as having ultrahigh permeability. Of the three well pairs involved in this test, one of them indicated very high interwell transmissibility. We believe that this high permeability is associated with a localized thin facies near the top of the reservoir, which has not been described geologically to date.

For the six well pairs comprising the two tests discussed here, interwell distances varied from a minimum 2,921 ft (890 m) to a maximum 4,499 ft (1371 m). The observed time lags and pressure responses varied from 72 to 1,728 minutes and from 0.12 to 7.05 psi (0.83 to 48.6 kPa), respectively. First pulse responses and first inverse pulse responses were recorded in three well pairs. For the other three well pairs, second pulse responses were recorded also.

Reservoir Description

The reservoir tested and described in this paper is in the Arab-D formation, the major productive formation in the Ghawar field. This formation is a massive carbonate structure grossly subdivided into five separate zones. The reservoir of interest comprises interbedded layers of limestone and dolomite, and it is quite heterogeneous both vertically and areally. In general, there is a degradation in both porosity and permeability vertically with increasing depth in the reservoir. Some vugular porosity occurs throughout the reservoir, in both limestone and dolomite facies.