A Novel Technique to Utilise Surface Data for Pressure Transient Analyses in Gas Producers

Petroleum engineers rely on periodic acquisition of bottom-hole data to monitor changes in well and reservoir inflow characteristics, and to quantify well and reservoir parameters using Pressure Transient Analysis (PTA). However, the acquisition of this data, especially in an offshore high pressure and sour gas environment, presents many logistical challenges and operational risks, and can negatively affect expenditure and downtime performance. On the other hand, surface data is routinely available, sometimes with sufficient frequency and precision to carry out PTA. Surface data is affected by wellbore transients that must be taken into consideration for converting the surface data to bottom hole conditions with the accuracy required for proper analysis. This paper describes a novel technique to properly account for wellbore transient effects and convert surface data to downhole conditions to enable conventional PTA without the need for well intervention. The technique combines the transient wellbore modelling with a novel approach of defining coefficients in a modified bottom hole pressure equation which is described in the paper. Coefficients are calibrated using concurrent surface and downhole data sets and then utilised to convert future surface data to downhole conditions. The paper describes the findings and conclusions from a five-well pilot, which involved wells with a range of reservoir characteristics, single and multi-layer production, and with/without cross-flow. The pilot work compared PTA results utilising this technique and surface data with results generated using downhole data.