Some Pulsed Neutron Logging Contributions To Improved Formation Evaluation

Abstract

Pulsed neutron logging, because it can be performed in cased wells, is supplying previously unobtainable information for evaluating reservoirs. When spaced in time, these logs make it possible to deduce the nature of water encroachment and hydrocarbon depletion at any stage in the producing life of a well. Introduction Hydrocarbon volume and geometry within a reservoir must be determined before the efficiency of hydrocarbon production can be judged. This requires a more thorough analysis than is possible during the early development of a field. During this early period many assumptions (universally based on incomplete sampling) are made for a number of factors including water saturation, reservoir boundaries, average reservoir thickness and ultimate recovery efficiency. Data obtained by initial drilling, well logging and production tests are the primary tools for early evaluation of the reservoir. Production history is inadequate in the initial stages of pool development, but as a field is developed and more wells are logged, more data are collected, and useful production history accumulates. The production trends may change depending upon rate of production, drive mechanism, state of reservoir development, porosity and permeability distribution, and efficiency of completion methods. All in all, production history is regulated by a complex set of conditions, some of which have been rather arbitrarily established by the engineer. It is commonly assumed that the formations exposed by the wellbore of the discovery well are representative of the subsurface of the entire location (or lease). Each log of a subsequent well will measure the physical properties of the formations immediately surrounding that particular well. The formations exposed are generally thought to extend laterally and continuously to connect with counterparts in adjacent wells. Preferential watering-out of individual strata within a formation commonly occurs. Pulsed Neutron Logging Pulsed neutron logging (PNL) has been well described in the literature.1-6 It is relatively insensitive to commonly occurring changes in borehole size and fluid type,7 and is well suited for use in cased wells. The casing of a well before "open-hole" logs are run no longer precludes the high probability of efficient formation (and well) evaluation by logging methods. The resulting cased-hole logs, when used with companion services such as neutron or interval-time logging through casing, will supply much the same information as is obtained from standard open-hole logging programs.