Studies of Producing Reservoirs With the Neutron Lifetime Log
Authors: Marquis G.L. 
Published: December 2007

MARQUIS, G.L., WICHMANN, P.A., JUNIOR MEMBER AIME, MILLIS, C.W., LANE. WELLS CO., DIV. OF DRESSER INDUSTRIES, INC., HOUSTON, TEX.

Abstract
Reservoir evaluations are made on information obtained at the time wells are drilled and completed and on the basis of reservoir performance. Important data normally available from open hole measurements include gas-oil contacts, water saturations and water table levels. These parameters change as a reservoir is produced and the results manifest themselves as changes or a combination of changes in oil produced, water produced, gas-oil ratio or bottom-hole pressure. The Neutron Lifetime Log gives a cased hole measurement of a formation parameter sensitive to the amount and type of fluid in the formations. This parameter-thermal neutron capture cross-section - qualitatively distinguishes potentially productive intervals from non-productive intervals under a wide range of wellbore conditions. Well studies are presented indicating results observed from applying cased hole measurements of capture cross-section to reservoir evaluation.

Introduction
The Neutron Lifetime Log uses a pulsed accelerator neutron source to measure a specific formation characteristic, the thermal neutron capture cross-section. Every element has its unique capture cross-section for thermal neutrons. As a result, each combination of formation and contained fluids penetrated by the borehole has its own particular composite thermal neutron capture cross-section. Where the contrast in capture cross-section properties of the contained formation fluids is significant, and where fluids are found in sufficient quantities, this phenomenon can be exploited to determine water saturation. The log can be run in either cased or uncased holes. Obviously, because of the lack of such a tool in the past, the greatest application has been in cased holes, primarily to determine the feasibility or need of workover operations due to occurrences such as bypassing of oil, normal depletion or offset drainage. Because of its success in these areas it is logical that the measurements can and should be applied to full scale reservoir studies, either as a complement to openhole logging data or as a method of continually updating various reservoir parameters such as fluid contacts, much as periodic measurements are made to determine changes in reservoir pressure.

Thermal Neutron Capture Cross-Section
Capture cross-section is a function of neutron energy and can be defined only for a specific energy. Neutron capture cross-section of a medium can be defined as a statistical probability of capture of that energy neutron in the medium. It is a convenient and useful analogy to liken this statistical probability to an area which target nuclides present to the moving neutrons. The term thermal neutron refers to any neutron in thermal equilibrium with its surroundings and identifies the neutron energy level or velocity. At 20C, the most probable thermal neturon velocity is 2,200 m/sec. The macroscopic thermal neutron capture cross-section for a cubic centimeter of material is the arithmetic sum of the capture cross-sections of each individual nuclide. This is expressed either as cm2/cm3 or barns/cm3 with a barn the equivalent of 10(-24) cm2. Table 1 shows the thermal neutron capture cross-section of several elements commonly found in the strata of sedimentary basins. The magnitude of the cross-section of chlorine with respect to the other most common elements present in formation waters and hydrocarbon fluids forms the basis of a quantitative determination of water saturation. Since the composite thermal neutron capture cross-section of any medium is based on the absolute finite number of nuclides of all its constituents, this quantity as applied to a fluid reservoir can be expressed as:
TABLE 1-THERMAL NEUTRON CAPTURE CROSS-SECTIONS.
Element a barns/atom Boron 750.0000 Lithium 070.0000 Chlorine 032.0000 Sodium 000.4900 Sulfur 000.4900 Calcium 000.4200 Hydrogen 000.3300 Silicon 000.1300 Magnesium 000.0590 Carbon 000.0045 Oxygen 000.0002
JPT
P. 412ˆ