Successful Deployment of EM Telemetry and Deep Reservoir Mapping in Underbalanced Drilling Environment, A Case Study from Pakistan Onshore

Abstract Major reservoirs in onshore Pakistan field are hosted in a gas-bearing limestone formation, with high depletion state up to 400 – 600 psi due to widespread production since the 1960's. Conventional overbalanced drilling (OBD) techniques were applied for these fields since their initial exploitation. During production, the quality of these highly fractured reservoirs has been decreasing due to invasion damage. Time-consuming activities for curing losses was one of the main reasons, among others for borehole issues (e.g. stuck pipe). This led to the implementation of underbalanced drilling (UBD) operations, starting more than a decade ago. Horizontal drilling was also employed to effectively produce only from the optimum zone. However, these methods had limited the capabilities for any technology utilization while drilling, mainly rooted on the real-time data transmission challenges towards the Air/Nitrified Foam UBD mud system. By compiling solutions from a drilling and geoscience aspect, this paper elaborates a case study for maximizing the usage of LWD solutions in UBD operations. Accurate real-time well placement was the main strategy to drain the optimum zone in this subject well, followed by acquisition of Borehole Imaging and Acoustic Sonic data from LWD technology to gain full carbonate reservoir insight with limited invasion. The steps for deployment of geosteering and LWD technologies in 8.5″ hole section are elaborated in the pre-job planning phase, real-time modelling and post-well review. The main challenge was to deploy technologies in a UBD environment, in which the telemetry for real-time data transmissions were optimized utilizing EMT (electromagnetic telemetry). Feasibility studies in pre-job stage were conducted to ensure EMT's signal strength was reliable to counter the propagation medium of air/nitrified foam mud system. Landing and laterals were conducted in the same hole section utilizing RSS (rotary-steerable-system) with ABG (at-bit-gamma ray) and 6-3/4″ OD Azimuthal Deep Resistivity with basic inversion modelling capabilities. Steering and mapping was tightly monitored as the drilling started from exiting 9-5/8″ casing shoe. Inversion enabled mapping the high resistive zone of Main Reservoir-B below the wellbore which led to identifying the landing point earlier than planned by ~2 m TVD below trajectory, ~105 m.MD before penetration by the bit. Top Boundary of Main Reservoir-B target zone (which was the conductive zone) was continuously mapped during the entire lateral section, providing high confidence in placing the well in optimum zone. At TD, a dedicated wipe run was conducted with 6-3/4″ OD Azimuthal Focus Resistivity for Borehole Image and 6-3/4″ OD Acoustic Sonic tool. Real-time reliable data transmission from EM Telemetry had secured the success for both geosteering and wipe runs. Geosteering enabled successful placement for a total drain length of 872 m.MD in the optimum zone, adding 300 m to the original plan. Smooth and vivid results from borehole imaging and sonic data aided the full reservoir analysis. This Subject Well's case had proven the comprehensive solution of maximizing LWD fleet in UBD operations, which enabled the completion to be run effectively. A novel standard has been applied in this field, the subsequent horizontal wells have adopted the same method.