The Methods of Correcting NMR Data from Dry Gas Wells Drilled with Formate Based Muds by Advanced NMR Techniques

摘要

Formate-based drilling mud is used to improve the performance of drilling and completions of high pressure and high temperature (HPHT) wells due to its properties such as high density (up to 19 lbs/gal), non-corrosiveness, eco-friendliness, high stability, and miscibility with brine and other fluids in the drilling muds. However, these strengths of formate muds pose challenges for formation evaluation due to their unusual properties such as high density, low Hydrogen Index (HI), miscibility with connate brines, and high conductivity. The current work addresses challenges of Nuclear Magnetic Resonance (NMR) logging, which is known to provide capillary bound fluid for producibility evaluation and accurate porosity for reserve estimation, in deep dry gas wells drilled with formate mud. With a carefully designed workflow composed of a suit of laboratory NMR techniques, the needs for the correction of T2cutoff and HI by Formate Mud Filtrate (FMF) invasion have been investigated. To quantify the accurate amount of correction required, FMF invasion has been monitored under reservoir conditions. The experimentally determined NMR T2 cutoff values for FMF saturated samples are always slightly longer than those for brine saturated ones. The difference of bulk volume movable (BVM), however, is less than 5% when T2 cutoff value from brine saturated sample is applied to FMF saturated sample. Thus, T2 cutoff determined by conventional NMR laboratory measurement with brine can be applied to NMR log with the formation invaded by FMF. The HI correction factors for BVM and bulk volume irreducible (BVI) are dependent on pore structures. For brine saturated bi-modal sandstones, all pores are displaced by FMF which corresponds to an HI correction factor of 0.74 for both BVM and BVI. For brine saturated carbonate with tri-modal pore system, however, 100% BVM and 70% of BVI have been displaced with FMF which correspond to HI correction factor of 0.78 and 0.80 for the FMF used in the current study, respectively. The outcome of the current study will help to enhance the usage of NMR logging as powerful formation evaluation tool for gas wells drilled with formate based drilling muds.