Effectiveness of Geostatistical Method in Converting Time Map into Depth Map - A Case Study in MHS Area (2006)

Summary The Mumbai High L-III reservoir is a highly heterogeneous multilayered. Carbonate reservoir lying on the continental shelf of western India in the Arabian sea. The layer thickness varies between 3m to 8m. As a part of ongoing redevelopment plan, multiple horizontal wells with drain holes of the order of 500m are being drilled through these thin productive layers. It is a challenge to keep the well bore throughout in these thin layers. Detailed drilling plans are made in advance along the drain hole. The plan is made on the basis of projected well trajectory in terms of deviation in True Vertical Depth (TVD) along the drain hole. Logging While Drilling (LWD) technology is used to keep the well inside sweet zone. Depth profile is very crucial to optimal completion of the well as drilling starts off in the direction as envisaged in the drilling plan which takes into various drilling constraints like Dog Leg severity etc. As the drilling progresses the real time LWD data, being monitored at Virtual Reality Centre in Mumbai city helps to keep the well inside sweet zone. Monitoring team intervenes as and when poor facies are encountered on the well log. The intervention is in terms of either losing or gaining the TVD for further drilling. The guidance comes from the depth map supplied to them. This depth map has been prepared for entire Mumbai High South (MHS) area which is roughly 900 sq. km. By integrating seismic time pick, well depth pick of corresponding horizon, and well driven and seismic driven velocities, it has been found that sometimes following the well trajectory based on this depth map takes the well into the over and underlying shale layers and mid course correction is applied to keep the well in the sweet zone. This deviation of the drain hole reduces its effective length and causes other drilling and well completion problems. On closer scrutiny of such cases it was found that the deviation from actual was prominent where the model suggested deviation in terms of TVD by more than 8m. When the trajectory was superimposed on seismic time section the time map was not supporting this. This gave rise to the doubt of having wrong velocity gone into converting time map to depth map. Keeping this in mind an alternative approach using Geostatistics was applied to prepare depth map on Localized basis so as to give proper weight age to the correlation coefficients valid for that area. A small area of the order of 25 sq. km. was taken around the well bore to be drilled. The depth picks from the existing wells falling that area and corresponding time horizon was input for geostatistical analysis. Since depth conversion is a non-stationary problem, kriging with external drift was used. Based on this this depth map the well trajectory was prepared and then tested against the actual TVD profile of the well path being drilled. It was found that it was closer to it compared to the one prepared on the basis of well and seismic velocities confirming its effectiveness. It helped RTO team in taking proper decision and thereby saving valuable Rig time. The well completion also was more near to optimum.