Geothermal well design using the new 2015 New Zealand standard and 1991 standard: A case of MW-20a in Menengai, Nakuru county, Kenya

Abstract

The design of geothermal wells is an important task in well construction, as this allows the effective conveyance of geothermal fluids from deep depths to the surface for utilization. The New Zealand design code NZS 2403:1991 has been used for the last two decades to design geothermal wells, but in 2015 it was replaced by a new design code NZS 2403:2015. This report presents the design of a geothermal well using the two design codes for comparison purposes. The well was designed using exact reservoir conditions in Menengai geothermal field in Kenya, using MW-20A as a reference well. After determining the reservoir pressure, the minimum casing depth for the different casing strings was determined from the codes. In addition, the design premises were established and it was found that the worst case for design was when the well was considered to be filled with steam from bottom to surface. Design calculations were carried out given this condition using the two codes and the best casing strings were determined. The design computations showed that a 20” 94 lb/ft casing, 13⅜” 54.5 lb/ft casing and 9⅝” 47 lb/ft casing were adequate to be run in hole for surface, anchor and production strings respectively

Further calculations showed that the weight of the production casing could be reduced to 36 lb/ft and still be within the minimum design factors, but to account for corrosion during the life of the well, the 47 lb/ft production casing was selected. Due to high stresses when the production casing rises into the wellhead, the weight of the upper two joints of the anchor casing string was changed to 72 lb/ft from 54.5 lb/ft. Several design considerations have changed between the codes, that include the following: The 2015 code gave deep minimum casing depths compared to the 1991 code. Temperature reduction factors for yield were reduced in the 2015 code when checked against the 1991 code. Minimum design factors for thermal expansion of anchor casing into wellhead and compressive stress in liners were reduced to 1.4 and 1.0 in the 2015 code from 1.5 and 1.2 in the 1991 code, respectively. The 2015 code considers fracture pressure for maximum pressure boundary while determining minimum casing depth while the 1991 code considers the overburden. The 1991 code does not allow the thermal expansion of the casing to exceed minimum yield, while the 2015 code acknowledges the use of strain based design in such cases. It is recommended that to avoid the introduction of tensional stresses into the well, cold fluids should not be pumped into a hot well.