A Laboratory Study of Nuclear Magnetic Resonance Relaxation and its Relation to Depositional Texture and Petrophysical Properties — Carbonate Thamama Group, Mubarraz Field, Abu Dhabi

Abstract

ABSTRACT We performed laboratory measurements to deter= mine whether NMR logging would improve log evaluation of the Mubarraz Field. The Mubarraz Field is a carbonate field in offshore Abu Dhabi, that produces from the Cretaceous Thamama Group. Existing log evaluation using classical logs has some shortcomings. Some zones that are not highly resistive actually produce water-free oil. This is because Mubarraz samples often have a large amount of mi-croporosity. The microporosity occurs because samples are highly micritized, and grain-sized particles are generally peloidal with internal microporosity. The microporosity holds a large volume of capillary-bound water, which results in low resistivities in zones that still have producible oil in large pores. An additional evaluation problem is that permeability varies widely while porosity remains almost constant. As a result, permeability vs. porosity correlations are less valuable than normal. In laboratory measurements on Mubarraz samples, we found that NMR relaxation measurements can provide: good estimation of the capillary-bound water volume at an air/water capillary pressure of 25 psi, using a cutoff on NMR T2 relaxation time of 190 ms.good identification of the grainstones as opposed to the lower-permeability textures (packstone … mudstone) with a T2 threshold of about 225 milliseconds.estimates of permeability that are significantly better than can be obtained from porosity alone. NMR relaxation measurements contain information about the pore size distribution, and it is this information that permits estimation of production-governing parameters. The estimators are based on parameters that can be obtained from continuous NMR logging. We compared NMR measurements with a synthesis of laboratory measurements that characterize micro-geometry. The synthesis is based on modeling wackestones as packed near-spherical particles of equal size. Values computed from this model for permeability, surface area and mercury injection agree with measured values. This model is a useful starting point for more complex models. We also observed that lower values of cementation exponent m were associated with higher dolomitexalcite ratios and shorter NMR relaxation time T2. The variations in m are large enough to affect interpretation of water saturation from resistivity logs. We tentatively ascribe these associations to the presence or absence of dolomite rhombs which fill solution pores. Solution pores are generally accompanied by high cementation exponents. When they are large, solution pores exhibit NMR relaxation times that are large. Their long relaxation times make interpretation of NMR logs more difficult, because they are indistinguishable from the long relaxation times of intergranular pores that are associated with high permeability.