Abstract

Properly designed, conducted and analyzed transient tests can be used to determine reservoir behavior and well conditions and estimate reservoir properties needed for field management. Testing the Yacheng 13–1 wells provides a case history for achieving these objectives under conditions of high temperature. The Yacheng wells encountered natural gas at a temperature of 350 F (177 C) and a pressure of about 5600 psi. This paper discusses the testing process and the experience of ARCO China in testing the Yacheng 13–1 development wells. This experience shows the key items needed to assure a successful test in a difficult environment.

The key items are determining the test objectives, designing a test that would achieve these objectives, estimating the pressure and temperature of the well during the test, selecting the proper tools and equipment to obtain the needed measurements, carefully coordinating the tasks between operating and service companies personnel, monitoring the test data in real-time to change the test procedure if needed, analyzing the data to estimate the well/reservoir system properties and finally, using the results to update the testing procedures for subsequent wells.

The tests performed included modified isochronal, drawdown and buildup tests. Tests had to be designed such that PVT-quality samples could be obtained to help design the production and transportation facilities. Carefully designing and conducting the tests, aided by the ability to perform real-time analysis to alter the testing procedure, enabled the operator to obtain the needed information about the well/reservoir system. The paper shows how the information is intended to be used in managing the field.

Background

The Yacheng 13–1 Gas Field lies in 300 ft of water, approximately 60 miles south of Hainan Island in the South China Sea. Figure 1 shows the location of the field. The field occurs along a structural ridge known as the Yacheng Spur which extends south-eastward from the Hainan Shelf plunging downward in a step-like fashion across numerous fault blocks. Six exploration and appraisal wells were drilled to delineate the field in the 1980's. The A1 well is the first development well to be drilled and is located at the top of the structure. Figure 2 shows the locations of the previous exploration wells 13–1–1, 13–1–2, 13–1–3, 13–1–4, and 13–1–6 as well as the location of A1. The Yacheng 13–1 is a combination structural and stratigraphic trap that is filled to the spill point. The Late Oligocene/Early Miocene sandstone reservoir units in the field onlap a basement high (the Yacheng Spur) to the west and are truncated by a regional unconformity (Figure 3). The pay zone in the reservoir is the Lingshui III or L3 (formerly named by ARCO as the Yacheng 13–1 Sand). The L3 sand is a very coarse sandstone at the top of the section and becomes finer grained in the lower portions. Porosity and permeability increase upward within the L3 interval. The reservoir is capped by a shoreline sand body that reaches 100 feet in thickness. The average net formation thickness is about 400 ft with a Net to Gross ratio of about 0.85. The average porosity is about 0.14. The reservoir temperature is relatively high (about 350 F) which necessitates the use of high temperature gauges in well tests. Several drillstem tests (DST's) were run in the exploration wells.

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