Hydrocarbon Saturation Determination in Case of Total Losses: Invasion Profile Modelling with Dual Resistivity – A Possible Application in PMCD Drilling

Formation evaluation and appraisal in Central Luconia carbonate reef is challenging when drilling operation change from conventional drilling to pressurised mud cap drilling (PMCD). PMCD drilling has always been the choice to deal with unmanageable losses condition. It is normally applied in carbonate reservoir with karst and vugs. Under this drilling condition, annular pressure and surface pressure is maintained above the formation that is able to take the cuttings and fluids. At the same time, light annular fluid is pumped down the annulus to maintain hole fill and avoid gas migration. Seawater, acts as sacrificial mud is pumped down the drill string to cool the bit and to transport the cuttings to loss zones. Meanwhile, for an exploration well, the primary objective is to prove hydrocarbon presence and hydrocarbon fluid contact through logging while drilling (LWD) as wireline logging is not favourable from operational perspective. One of the key challenges of interpreting hydrocarbon saturation in PMCD operation is suppression of resistivity value due to sea-water invasion. Indeed, with PMCD, the well is appeared to have high water saturation even though 1 MHz phase shift 36" spacing deep phase resistivity is used in the interpretation. However, this is inconsistent with gas kick occurred at the top of carbonate or gas shows prior to conversion from conventional drilling to PMCD operation. Another observation of resistivity log response in PMCD drilling is that the phase shift resistivity from different sensor spacing (6", 12", 24" and 36", with smaller number indicate shallower depth of investigation, and higher number indicate deeper depth of investigation) appears to have separation, which indicates invasion profile which happens at one time-frame. Although 1D inversion for true resistivity (Rt) can be carried out with multiple sensor spacing phase resistivity and invasion diameter (Di) as inputs, the inversion result does not yield satisfactory result that match pre-PMCD resistivity value. The objective of the paper/ abstract is to highlight the benefits or running dual – resistivity in LWD bottom-hole assembly (BHA) in PMCD well to capture time-lapse resistivity measurement, estimate Rt which is time-dependant and pin-pointing gas-water contact in the exploration/ appraisal wells. This new proposed concept and methodology is still at its early stage, yet designed to make better decision during operational time. Such an approach will provide benefits to petrophysics community in the PMCD well interpretation with minimal incremental cost.