A New Technique for Testing Shallow High-Rate Gas Wells Using a Deep-Set Surface-Controlled Tubing-Retrievable Subsurface Safety Valve
Authors: Prise G.J. , Davidson A.R. , O&apos;Callaghan M.J. 
Published: October 2008

This paper demonstrates how good transient well test data can be successfully obtained from shallow, high rate, gas production wells. Downhole shut-in has been reliably achieved on wells producing up to 82 MMSCF/D. The innovative use of standard completion equipment in testing these wells has made a valuable contribution to quality data acquisition and has been of great benefit in predicting long term production performance.

INTRODUCTION

Engineers continue to be faced with new challenges in the field of well testing. Many problems are solved by designing new test tools. However, in some cases solutions can be found by enhancing existing tools for use in new applications.

Hamilton Oil Company Ltd (HOC) have successfully performed a number of well tests on some shallow gas wells. The problem faced was obtaining high quality pressure transient test data from wells in a relatively low pressure, highly permeable gas reservoir. Conventional downhole shut-in tools were found to unacceptably limit the production flowrate and hence the reservoir drawdown. Large diameter tubing with minimum restrictions and downhole shut-in were required.

HOC identified the novel technique of using a large diameter, deep-set surface controlled tubing retrievable subsurface safety valve (TRSV) for the dual purpose of safety and downhole shut-in. HOC worked with a world-wide well servicing company to increase the depth setting capability of a conventional TRSV, which is normally used in production well completions.

This paper provides a reference for the engineer designing a shallow high rate gas well test. Details of the test objectives and the various test string designs are given. Design and modifications to the TRSV for this specific application are discussed. Test results from successful case histories are presented.

TEST OBJECTIVES

A total of six welltests were performed. The objectives of these welltests were as follows:Perform testing operations in a safe and cost effective manner.Determine permeability-thickness, skin, reservoir pressure and reservoir boundary effects.Obtain representative reservoir ffuid samples.Establish whether there was potential for sand production or water coning during high rate gas flow.

In order to fulfil these objectives, both high gas flowrates and downhole shut-in are required as part of the welltest programme.

CONVENTIONAL TEST STRING DESIGNS

Standard DST String

A standard DST tool string consists of a number of annulus pressure operated downhole test tools as shown in Figure 1. This string design provides the following main features:Downhole shut-in to minimise well-bore storage effects during transient pressure build-up tests.A mechanical barrier for tubing pressure testing or downhole shut-in, in the event of an emergency.A simple means of circulating for well killing.

The main disadvantage of this test string is its limited bore. Standard test tools, as shown in Figure 1, have an inside diameter of 2.25 in. In gas wells, this restriction causes very severe pressure drops when flowing at high rates with low bottom-hole pressures. The vertical lift performance of this test string was modelled on two independent computer models for Test NO.1.