Well Test Analysis Of HGP-A
Authors: Chen B.H. , Kihara D.H. , Seki A. , Yuen P.C. 
Published: December 2007

Since the successful initial flashing on July 2, 1976, HGP-A has undergone five flash discharge tests with the longest one lasting 42 days. Production records including wellhead pressure and Production records including wellhead pressure and temperature, production rate and steam quality were kept for drawdown analysis. After two of the discharge tests, Kuster pressure bombs were lowered to the bottom repeatedly to record pressure data for buildup analysis.

Initial analyses of the drawdown and buildup tests indicate that the Kapoho Geothermal Reservoir, where HGP-A is located, seems to be in a tight formation with possible severe mud damage in the well. The reservoir also appears to be a liquid-dominated system but with two-phase flow during the discharge of HGP-A. Evidence of this claim will be presented in this paper.

The pressure drawdown art buildup analyses were performed with the traditional single-phase petroleum reservoir engineering techniques petroleum reservoir engineering techniques modified when necessary.

Introduction

The experimental well, HGP-A, drilled under the auspices of the Hawaii Geothermal Project, is located on the island of Hawaii near the eastern rift of Kilauea volcano. Drilling was completed to a depth of 6450 feet (1966 m) in April 1976. The well is cased to 2230 feet (680 m) below the wellhead, which is approximately 600 feet (183 m) above sea level. A slotted liner is placed from the end of the casing to bottomhole. Cuttings and core samples obtained during drilling indicate that the region is composed of volcanic basalt with a profile that contains open fracture zones separated by relatively impermeable layers.

The well has undergone five flash discharge tests since an initial flashing on July 2, 1976. Figure 1 is a sketch of the equipment and instrumentation for the discharge tests. As shown, the method involves basically the James technique for measuring total mass flow with twin cyclone separators for separation of steam and water. A 90 degrees V-notch weir is used to measure the liquid flow rate, permitting steam quality and specific enthalpy to be calculated. A recovery tube is mounted on the wellhead to permit temperature and pressure profiles to be obtained and water samples pressure profiles to be obtained and water samples to be gathered during quiescent and discharge periods. periods. Since the temperature of the reservoir in general exceeds 300 degrees C (572 degrees F) with a maximum recorded temperature of 358 degrees C (676 degrees F), no electronic equipment can survive the extreme conditions downhole. Therefore, Kuster Amerada RPG-3 Type subsurface recording temperature and pressure gauges were selected to provide all temperature and pressure measurements downhole.

DOWNHOLE FLOW CHARACTERISTICS

During January and March 1977, the flow tests consisted of a series of discharges in which the flow was throttled by placing orifice plates of various sizes in the discharge line. The results are summarized in Table 1. Pressure and temperature profiles taken during the throttled flow tests are shown in Figures 2 and 3. These profiles indicate that the fluid in the wellbore is profiles indicate that the fluid in the wellbore is at saturation conditions with a mixture of liquid and vapor flowing up to the wellhead. Since the steam quality at the wellhead is high and no steam/water interface is found in the wellbore, the conclusion is that flashing occurs in the formation rather than in the wellbore.

Examination of Figure 2 shows that the pressure profiles are essentially three constant pressure profiles are essentially three constant slope lines meeting at the junction of the casing and the slotted liner and at approximately 4300 feet (1311 m).