Aerated Underbalance Drilling Screening Assessment at a Geothermal Field in Indonesia

Abstract

Fault network is a challenging problem for drilling operations. Formation fluid contains high temperature production fluid which can reach more than 225oC on high enthalpy system. The other consequecies 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 periode. 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 enthalphy 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 fomation pressure and conservation areas are problems for geothermal drilling. Underbalance drilling method has an advantage dealing with low pressure reservoir. Loss circulation problem can be mitigated by the drilling fluid pressure which is intentionally lower than the reservoir pore pressure. This allow reservoir pore pressure to push the fluid upward to the surface, thus canceling the probability of drilling fluid to seep thru the formation. Along with the advantage, underbalance drilling comes with some weakness. Low reservoir pressure makes a limited feasible bottom hole pressure window area. High temperature reservoir makes the problem even complicated. High temperature makes the hole cleaning ability become much lower. Drilling fluid viscosity comes lower as the temperature gets higher, thus the ability of drilling fluid to hold the cutting decrease. Underbalance drilling offers great advantage as well as some disadvantage that has to be considered. Underbalance drilling with it’s advantages and weaknesses has to be studied. This study will help the quantitative and qualitative decision wether the underbalance drilling is feasibel 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.