Numerous cases of loss of zonal isolation following acidization of oil wells in certain fields in Brazil have been observed, even in wells which have been recemented. As a result, a project was undertaken to quantity the solubility of cement samples in acids and to study the effects of these acids on the zone isolations which had originally been obtained by the primary cementing. Laboratory and large-scale apparatus were developed for static and dynamic testing. The factors which influence reactions between the Class G cements and different acid solutions used in Brazil were investigated. This led to the development of a laboratory procedure which proved to have excellent repeatability. Following the laboratory testing, the damage caused to cement samples made with normal additive formulations and with latex-based cements was investigated.
The large-scale dynamic simulator was designed to simulate the sequence of conditioning the well, the primary cementing, perforating and then the continuous injection of acid solutions. This apparatus enabled the cement/casing and cement/formation interfaces to be visually inspected, as well as the integrity of the cement sheath after continuous injection of acid. The experimental results and the field operation records were then used to develop recommendations for the optimization of both cementing and acidizing operations to minimize the occurrence of loss of zone isolation following acid treatments.
Introduction
There has been a lot of controversy regarding the effects of acid on cement, both on primary cementings and on the cement nodules formed by squeeze cementings. The predominant oilfield point of view has been that the reaction is superficial, occurs in a short time period and that its effects are minimized by the formation of a protective coating which inhibits or prevents the reactions from continuing. According to this theory, the risk of an acidization breaking-down squeezes or causing loss of zone isolation by dissolution of the cement would be minimal. Also, the area of cement exposed to the acid at the perforations would be very small. However, may field reports show that either the cement sheath or the interfaces are damaged by the acidization. Acoustic logs run before and after acidizations often confirm loss of adherence. In other cases, injection tests into zones which had previously been hydraulically isolated show intercommunication following the acidization.
Blount et al. recently published a study on the effects of acidization on cement nodules and recommended the use of latex to reduce the acid-solubility of cement. As latex cement formulations are significantly more expensive than conventional formulations, the present study was commissioned to look principally at primary cementings and to identify the causes of loss of zonal isolation after acidization and then to properly evaluate the efficiency of latex as a means of avoiding the dissolution of the cement.
Case Histories
The effects of acid attack at the interfaces of wells in the Serraria field have been very well documented by different types of acoustic bond logs run before and after the acidizing operations, as shown by Silva et al. To reach the planned water injection target, well SE-10, for example, was selectively acidized with 15% HCl and with a 12% HCl 13% HF mixture. The logs showed loss of bond following the acidization. The well was squeeze-cemented and bond logs showed that good bonding had been re-established. A further selective acidization was performed and again there was interzonal communication and the acoustic logs showed loss of bond. A second squeeze was performed and the logs showed that bonding had again been re-established. However, due to the successive losses of zone isolation, the well was completed for continuous water injection in the two intervals due to the impossibility to obtain the minimum injection quotas for the project by acidizing.
Reservoir studies of the Canto do Amaro field showed the most promising secondary recovery method to be water injection to the main block of zone 630. A pilot project was implanted, with 4 wells recompleted with dual completions, acidized with 15% HCl and with 12% HCl / 3%HF. The planned injection quotas for the reservoir were obtained. However, in spite of this, the pilot project did not give the expected results.
During a workover in one of the 4 wells, intercommunication was detected between zone 630 and the upper zone, 615, possibly caused by acid damage. This was seen as a possible cause for the failure of the pilot project. P. 371
