Analysis of Pulsed-Neutron Decay-Time Logs in Acidized Carbonate Formations

Abstract

AL-SAIF, A.S., ARAMCO ABQAIQ, SAUDI ARABIA COCHRANE, J.E., MEMBER SPE-AIME, ARAMCO, DHAHRAN, SAUDI ARABIA EDMONDSON, H.N., SCHLUMBERGER TECHNICAL SERVICES, PARIS, FRANCE YOUNGBLOOD, W.E., MEMBER SPE-AIME, SCHLUMBERGER OVERSEAS, DHAHRAN, SAUDI ARABIA Abstract The measurement of thermal neutron decay times by means of pulsed neutron tools has become an important reservoir-monitoring technique. In many types of reservoirs, these measurements permit the location of oil remaining behind casing. A requisite condition for the application of this method is knowledge of formation porosity and chloride content. This knowledge usually is derivable from the open-hole logs run before completion of the well. However, when the producing zones are treated with hydrochloric acid, either of these parameters may be changed. This paper presents examples of dual-spacing thermal neutron decay-time logs in Arabia, where prior acidizing bas altered the log response to The prior acidizing bas altered the log response to The point of producing erroneous conclusions unless point of producing erroneous conclusions unless this effect is accounted for. A hypothesis is advanced explaining this phenomenon as the result of either or both the porosity increase created by acidization and the retention of chlorides from the acid by the formation. Although no way has been found to differentiate positively between the two effects, experience indicates that the cumulative effect observed on The decay-time log is permanent during the water-free productive tile of the well. Thus, the recognized production-monitoring technique, known as time-lapse decay-time logging, is still valid and useful providing that The original "reference" decay-time log is run after acidization. This paper investigates various aspects of the problem and details ways in which it has been problem and details ways in which it has been dealt with in practice. Introduction A dramatic acid effect on pulsed neutron decaytime measurements was recognized by the Arabian American Oil Co. (ARAMCO) late in 1973. Before this time, ARAMCO was successful in using periodic decay-time logs to monitor water-saturation changes in nonacidized carbonate reservoirs. During 1973, a number of logs were run in acidized wells in the Arab D reservoir of the Ghawar field for the purpose of detecting sources of water production. Results were confusing at best until a base log recorded in a clean oil producer revealed be acid effect producer revealed be acid effect Extensive inquires were made to shareholder companies, other Arabian Gulf operators, and to Schlumberger. It was found that, although acid effects had been recognized, no correction techniques had been devised. The only guideline given was to disregard water-saturation calculations in acidized formations. Since such calculations were the primary reason for running decay-time logs for monitoring, and most ARAMCO wells were acidized during completion, this guideline apparently left no alternative but to cease decay-time logging in carbonate reservoirs. Since there were no other techniques for water-saturation determination in cased holes, however, A was recognized that a workable solution to this problem had to be found. Early in 1974 a controlled evaluation program was begun to study the acid effect on dual-spacing decay-time measurements. The program considered the following questions. Is be acid effect truly caused by acidization of carbonate reservoirs? Is it a permanent effect, or does it disappear with oil or water production? What is the physical nature of the effect and can it be accounted for in water-saturation calculations? Can the anomalous behavior be used to evaluate the effectiveness of acid treatments? Dual-spacing decay-time logs were obtained in many wells, before and after acid treatments that displayed a variety of characteristics (such as, rates, volumes, concentrations, use of diverting agents, etc.). Also, open-hole porosity and resistivity measurements were obtained before and after treatment to study the effects of acid on other parameters. SPEJ P. 453