Better Well Performance Diagnosis and Reservoir Management with Alternative Well Outflow Prediction from Your Desk: Well Modelling-Based Approach at Shallow Zone of Tunu Field, Mahakam

Abstract

Shallow Zone of Tunu Field (TSZ) which was initially identified as drilling hazards has been extensively developed to produce gas from widespread and scattered gas-bearing sand reservoirs all over 1500 km² of deltaic area in Tunu Field with extremely good reservoir properties. It has been produced more than 600 Bscf with main reservoir characteristic: strong aquifer support with maintained reservoir pressure at hydrostatic. Water encroachment monitoring is critical for wellbore performance diagnosis and reservoir management. However due to operational limitations, water outflow measurement could not be implemented continuously: spot test performed by gathering separator, mobile testing unit concern, deltaic field environment, manpower, out of service equipment, etc. By having empirical observation, there is no clear relationship between variables: water outflow and wellhead flowing temperature (WHFT) for known variable of gas production rate (Qg). This uncertainty is coming from several parameters that affect WHFT: multiphase flow rate, geothermal gradient, pressure drop in the tubing, specific heat of the gas, etc.

By using wellbore temperature profiles approach that developed originally for predicting temperature profiles of multiphase flow  inside conduit, water outflow estimation could be estimated by reverse calculation of known variables. Well modeling-based approach has been initiated to predict water outflow based on available wellhead parameter: 1) Analytical Model; 2). Dynamic well modelling; 3) Steady-state well modelling. Model validation shows good performance in estimating water outflow. Water Gas Ratio - WHFT for Qg variation chart could be generated automatically by in-house tool for specific well or field basis in order to be used for practical operation. Another approach is proposed if Qg becomes unknown parameter/unreliable: Nodal Analysis coupled with wellbore temperature profiles approach by WHFP & WHFT matching at certain downstream operation condition (i.e. choke opening & flowline pressure mode). This “from your desk” approach could be an alternative for continuous/regular production test in order to estimate or predict well outflow prediction when facing operational constraints. The objective is to take better decision making for production optimization and reservoir management.