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The interest in exploring carbonate reservoirs has experienced a rapid growth in south-east Asia. However, significant drilling hazards have been encountered where karstification and fractures are present. This led to deployment of pressurised-mud-cap drilling (PMCD) to address total losses. PMCD involves drilling with no returns to surface, while drilled cuttings are injected back to the lost circulation zones. Under the PMCD conditions, wireline logging is usually ruled out, making logging-while-drilling (LWD) the only choice. PMCD often induces adverse borehole conditions such as enlarged hole and deep early time invasion, which affect log responses and challenge interpretation. PETRONAS has reported several wells in which well logs were badly affected when PMCD was activated. A recent carbonate gas well illustrates the impact on LWD responses of the deep invasion and borehole deterioration associated with PMCD. A multifunction LWD tool providing triple-combo logs, sigma, and spectroscopy measurements was combined with a high-resolution resistivity imaging tool to identify the gas-water contact. The hole was enlarged up to 10-inches, in contrast to the 8 ½ inch bit size. Reservoir evaluation was complicated by the following: Propagation resistivities read low in reservoir rock, erroneously suggesting a water zone; nuclear porosity (TNPH) was doubtful due to the effects of enlarged hole on the measurements; and there were no cuttings returns to confirm possible dolomite presence. To understand and discriminate formation response from the environmental effects, an integrated workflow was developed to provide information on rock texture and producibility. Even though low in absolute value, propagation resistivities with multiple depths of investigation separate, suggesting conductive invasion. True formation resistivity can be resolved by 1D inversion of the resistivity array. The derived Rt is close to the laterolog-bit resistivity from the LWD imaging tool in both absolute values and profile, indicating a gas column section. To estimate porosity, three methods were compared, with the assumption that the formation was fully invaded: (1) Resistivity porosity obtained from inversion of Archie's law; (2) sigma porosity and (3) TNPH. TNPH was corrected for lithology effects by using LWD spectroscopy calcium and magnesium yields to quantify calcite and dolomite. For resistivity based porosity, variation in Archie m plays a key role. The results showed TNPH and sigma porosities were consistent with Archie porosity for m varying between 1.6 (fracture-rich interval) and 2.4 (predominance of vuggy pores). These textural variations are observed on the borehole images. The work demonstrated the value of integrating multiple LWD measurements to reduce uncertainties caused by borehole degradation and carbonate heterogeneity under challenging PMCD conditions. |
