HGP-A reservoir engineering

Abstract

The Hawaii Geothermal Project well HGP-A has undergone a two-year testing pr.ogram which included cold water pumpdown tests, flashing flows with measurements of temperature and pressure profiles, and noise surveys. These tests and the data obtained are discussed in detail. While the pumpdown tests conducted right after the slotted liner had been installed and the mud removed indicated that the well had very poor permeability, HGP-A was flashed successfully on July 2, 1976. Maximum quiescent bottomhole temperature following that initial flash was measured to be 358°C. Comparison of subsequent discharges shows that with each succeeding test, the flow rate has increased, possibly due to the displacement of drilling mud embedded in the wellbore surface. The flow rates range from a maximum of 101 Klb/hr at wellhead pressure of 51 psig to a throttled 76 Klb/hr at 375 psig wellhead pressure, with possible electrical power production of 3.0 to 3.5 MWe. Temperature and pressure profiles taken during flow tests indicate that the fluid in the we1lbore is a mixture of liquid and vapor at saturation conditions. The absence of a liquid level during flashing discharge confirms that flashing is occurring in the formation. Pressure drawdown and buildup analyses yield a value of transmissibility (kh) of approximately 1000 millidarcy-feet with a pressure drop across the apparently damaged skin of 500-600 psi. The pressure profiles taken during flashing flow consist roughly of three approximately constant gradient lines that intersect at the junction of the casing and the slotted liner, and at approximately 4300 feet depth, which leads to the conclusion that the major production zones are near bottomhole and in the vicinity of 4300 feet. Furthermore, the data points on the log-log Horner type plot seem to fallon two different but consecutive straight-line approximations. This could be interpreted to be the result of two different production layers with different kh values.