Aerated Underbalance Drilling Screening Assessment at “X” Geothermal Field
Abstract
Fault network is a challenging problem for geothermal drilling operations. Formation fluid contains high temperature production fluid which can reach >225oC on high enthalpy system. The other consequences is that almost all fault network has low pressure or subnormal pressure. This low pressure results to a loss circulation problem. This low pressure can even go lower if the geothermal field has been exploited for a long period. A miss reservoir management, that do not re-inject sufficient amount of fluid, will cause the reservoir pressure go lower. Another problem in Indonesia is the conservation area which almost all high enthalpy geothermal system exist. The pay zone that is beneath the conservation area must be reached by directional drilling as a solution. High temperature fluid, low formation pressure and conservation areas are problems for geothermal drilling. To overcome these problems, underbalance drilling method has an advantage dealing with low pressure reservoir.
This paper introduces a way to screen the underbalance drilling method on a certain field. This study will help the quantitative and qualitative decision whether the underbalance drilling is feasible or not. The first phase qualitative decision is based on wellbore stability, loss circulation, reservoir damage, stuck pipe incident, hard drilling and cost benefit. Then it will go to the drilling fluid decision. And at the end as a quantitative decision for constructing a feasible bottom hole pressure window area with some hole cleaning assessment. Underbalance drilling assessment will be studied on field “X” at one of Indonesia’s geothermal field. The screening of “X” geothermal field comes with conclusions that it has an opportunity underbalance drilling can be implemented with vertical aerated drilling wells on spesific gas and liquid flow rates.
Keywords: Geothermal, underbalance drilling, aerated drilling
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