Integration of NMR with other openhole logs for improved formation evaluation

The recently introduced measurement of total porosity from nuclear magnetic resonance (NMR) tools can help to identify the hydrocarbon type and to imp rove the determination of formation total porosity (phi (t)) and water satu ration (S-wt) in combination with other openhole logs. In shaly formations, porosities are difficult to estimate in the presence of hydrocarbons, espe cially those for gas and light oils. Water saturations are even more diffic ult to estimate because critical parameters such as clay cation exchange ca pacities/unit pore volume (Q(V)), the formation factor (F) and formation wa ter resistivity (R-w) might not be known. The latter quantities are essenti al inputs into the Waxman-Smits and dual-water model saturation equations. In the typical case of shaly gas-bearing formations, both the total porosit y corrected for the gas effect and the gas saturation (S-x gas) in the flus hed zone can be derived by combining total NMR porosity (phi (NMR)) and den sity porosity (phi (density)) Adding resistivity logs such as R-x0 and R-t helps to differentiate between gas and oil. Furthermore, the flushed zone w ater saturation (S-xot) computed from I-S-x gas can be used in many ways. O ne procedure uses S-xot in conjunction with the R-xo saturation equation to determine Q(V) or F. Another technique uses S-xot in conjunction with the saturation point (SP) to estimate Q(V) when R-w is known. Yet, another meth od estimates ev directly from the NMR short relaxation time part of the T-2 distribution and use S-xot in conjunction with SP to estimate R-w. The new interpretation procedure follows the sequential shaly sands approach: firs t, determine porosity, second, determine shaliness, and, third, determine s aturation. The new procedure improves on the classical method by offering n ew ways to compute Q(V), F and R-w. The methodology is applied to a number of field examples.