Wellhead Shut-In Pressure Prediction Method to Minimize Risk: Unlocking Potential from Wells with Reservoir Pressure Above Surface Equipment Limit

Abstrak

Tunu is a mature giant gas field which located in Mahakam Delta. Recently it contributes 35% of Mahakam gas production and characterized with multi-layer reservoirs. Reservoirs with pressure above 6000 psia are observed at existing wells with depth deeper than 3800 mTVDss. These high pressure reservoirs will need special safety mitigation before put the well to production (i.e. X-mass tree pressure rating limit and flowline allowable pressure limit). A better methodology is presented in this paper to estimate Wellhead Shut-In Pressure (WHSIP) by considering buffer effect from existing depleted multi-layer reservoirs. It will give a more accurate WHSIP prediction and allow a better planning of perforation priority for operational purpose.

For safety consideration, WHSIP value is predicted without buffer effect. It could be performed for single layer reservoir by using pressure drop calculation along wellbore. For detail WHSIP calculation by considering buffer effect from multi-layer reservoirs, the calculation will be more complex. This paper demonstrates methodology and case experiences to predict detail WHSIP by using multiphase flow simulator for 2 model conditions: 1) Dynamic with fluid mechanics based model; 2) Steady state with Nodal Analysis based model. With high uncertainty of buffer zone data, both models being compared with error performance 10-15% compare to actual data. This result is considered reliable to predict WHSIP with respect to X-mass tree limit 6500 psi. Sensitivity case and also advantages-drawbacks of these models are described in order to create robust and "easy to use" model to be implemented for regular operation purpose. By implementing this method, risk can be predicted and prioritized to support production from reservoir potential with pressure above surface equipment limit. It will be useful for decision maker to consider perforation job from high pressure reservoirs with high potential possibility but still considering safety aspect.