Sand Production Prediction with Completion Selection for an Offshore Well with a Shallow Unconsolidated Reservoir Zone, Gas Field, Natuna Sea, Indonesia

Abstract

Abstract Sand or solids production is a concern during the production phase of a well. The process can cause several issues such as affecting and reducing the production of a well by plugging the wellbore, damaging downhole and surface equipment due to the erosive nature of the sand and, ultimately, effecting the economical state of a well. As such, sand production prediction studies are an integral part of overall field development planning so that sanding potentials can be assessed, and appropriate well completion and sand control designs considered. This paper presents a study case from the Natuna Sea in Indonesia to explain the importance of geomechanical modelling and a subsequent sand production prediction study in an offshore well with shallow and unconsolidated sandstone and shaly sandstone reservoirs with high fines content zones at ~300 m below the sea floor. The foundation of a sand production prediction study is the geomechanical model. The geomechanical model is specific to the area and characterises the current in situ stress state, pore pressure and formation rock mechanical properties. The model is built using offset well data including reports, drilling and completion data, well logs, measurements and tests and regional experiences. Following the estimation of vertical stress and formation pressure for the study wells, the full stress tensor is constrained as a function of depth. Ordinarily, the least principal stress is inferred from any leak-off (LOT) and formation integrity tests. In the case of this study, there aren't any valid data which meant that regional LOT data are used in the offset study wells. Further calibration was made using a mud loss event experienced during drilling operations in an appraisal well. The log-derived rock mechanical properties are calibrated with laboratory rock mechanical testing data from one of the offset wells. The maximum horizontal stress orientation and magnitude is modelled from the occurrence of stress-induced wellbore failure inferred from image data and calibrated with regional information. The final geomechanical model is verified against the drilling experiences of the respective offset wells. The calibrated geomechanical model is used to carry out the sand production prediction study. Information on the presence or absence of sanding events from several drill stem tests from the offset wells is used to develop a calibrated sanding model. A range of possible calibration factor values is used in sensitivity analyses, together with a range of well deviations and azimuths and various well depletion conditions, to determine the risk of sanding and understand the maximum drawdown pressure for sand-free production. The results show that the planned wells have a low to negligible well-life sanding risk for both open hole and cased and perforated completion This means that no immediate downhole sand control would be required for any of the planed wells. The results are interesting given that the target reservoir of the well is very shallow, and the sandstones are particularly weak and with intervals of high fines content. The results of this sanding risk assessment were used to optimise the trajectory and completion design of the upcoming development wells in the field.