Reservoir Management in South Louisiana With the Pulsed Neutron Log
Authors: Opsal S.J. 
Published: December 2007

This paper illustrates through case histories how the pulsed neutron log aids proper reservoir management in strong water drive reservoirs in south Louisiana. An economic analysis performed on the results of remedial operations using pulsed neutron logs clearly shows the benefits of risk reduction when compared with those remedial operations without pulsed neutron logs.

Introduction

The studied area, southwest of New Orleans (Fig. 1), provides an excellent illustration of reservoir management using the pulsed neutron log. The geologic structures in the area studied consist of deep-seated salt domes, which were delineated by reflection seismograph. The productive sands are of the Miocene and Pliocene ages. As wells go off production, remedial work must be performed either to initiate production in a new interval or to restore production to the same interval. The pulsed neutron log has become an integral part of the remedial work program in the studied area to ensure proper reservoir depletion. The first pulsed neutron log in the area was run in 1964, and since that time the yearly average number of pulsed neutron logs has risen to 35 to 40% of the total number of remedial operations. The log is being used to a large extent in the studied area due to the characteristics of the reservoir drive, which as a whole is a strong water drive. However, several sands exhibit a "bottom water" drive, which results in the coning of water to the perforations. This premature water break through limits the radius of depletion that can be achieved. Since the oil/water contact varies throughout the reservoir according to the extent of coning and amount of production, pulsed neutron logs are the most efficient means of monitoring the depletion of the reservoir and the movement of the oil/water contact. Other applications of the pulsed neutron log have been to ensure the proper depletion of membered sands to aid in the evaluation of low-resistivity sands, to identify the sealing nature of small faults, and to evaluate the effects of permeability barriers on recovery.

Theory

Neutron bursts with energy levels of 14 MeV are emitted every 1,000 microseconds. These high-energy neutrons are captured primarily by chlorine but also by the rock matrix, hydrogen, and materials in the wellbore. After capture of the neutrons, fixed-energy-level gamma rays are emitted from the capturing nucleus. It is these gamma rays that are counted during the 1,000-microsecond cycle. The number of capture events is proportional to the number of uncaptured neutrons at any time. Therefore, what is measured is the intensity and the rate of decay of the capture gamma ray flux. Many papers and articles have been written on the subject of evaluating pulsed neutron logs. The water saturation is calculated using this equation:

(1)

To calculate a quantitative SW, values for porosity, water salinity, shale content, and hydrocarbon type must be assigned or derived from the pulsed neutron log using nearby clean (nonshaly) water sands. After a water saturation has been calculated, a check for possible errors can be made by comparing the water saturation calculated from the pulsed neutron log with the original water saturation of the open hole electric log. A significant difference in these water saturations would indicate an error in calculation or the possibility of a depleted zone.

JPT

P. 419^