A Laboratory Investigation of Cementing Horizontal Wells

Abstract

Summary. Obtaining a successful cement job will remain one of the most important factors to the productive life of any well and will be especially critical for horizontal-well completions. Achieving a high mud-displacement efficiency under highly deviated or horizontal-well conditions requires that special attention be given to the many aspects of drilling/completion practices-e.g., drill-fluid systems and properties and casing and hole sizes-to obtain optimum mud displacement and cementing results. A study of factors affecting mud-displacement efficiency focused on cementing an ultralow-permeability formation that is being evaluated as a subject for horizontal completion. Realistic laboratory testing was conducted on a large-scale test model that has been used for many years to evaluate factors influencing drilling-fluid-displacement efficiency. Factors evaluated for this study included influence of hole and pipe sizes, pipe centralization, displacement rates, and spacer systems. Findings from this study provided specific recommendations for low-permeability reservoirs that also can be applied to horizontally drilled wells. Various techniques for cementing horizontal wells will be discussed and general recommendations will be given. Introduction Horizontal-well drilling is not a new concept. Interest in the possibility of greatly improved productivity from these wells vs. vertically drilled wells in the same fields has been evident since the early 1930s. Recently, the major area of application has been in regions where the production zones are very narrow. Drilling highly deviated or horizontal wells within very narrow production zones is done to maximize the contact area of the oil-bearing formations with the wellbore, thereby improving the overall drainage of the surrounding matrix structures. To date, however, few operators have cemented these long, horizontal sections. They have chosen instead to use openhole completion methods where tubing or slotted liners are the means of oil recovery. Recent research and field experience have brought into question the long-term integrity of such wellbores. Development of successful cementing practices under these conditions will increase the viability of such projects and create situations where each multiple-stimulation treatment along, the production interval can be isolated for maximum effectiveness. During the 1980's, there has been an increase in research concepts applicable to horizontal wells. The major area of investigation has been the development of drilling-fluid systems that would possess solids-transport characteristics such that compacted channels of these materials on the low side of the annulus could be minimized or eliminated. This research has shown that control of the rheological properties of the mud is critical in achieving the channel-free annulus required for a successful primary cement job. Crook et al. identified that a high yield point of the drilling fluid at ambient temperature (is greater than 28 lbf/100 ft2 [is greater than 13.4 Pa] controlled the settling of the mud weighting agent within a large-scale test facility under horizontal conditions. Others have identified the ratio of the yield point and plastic viscosity as the controlling factor that defines the solids-carrying capability of a drilling fluid. Okrajni and Azar also stated that the channel could be eliminated through circulation of the drilling fluid at turbulent rates. The research described here was initiated because of a growing interest in horizontal completions through the chalk formations of the North Sea area. These low-permeability chalk zones require stimulation to yield maximum production. The drilling fluid used in this work was an invert-emulsion, low-toxicity, oil-based mud designed for use in the North Sea region, where cuttings cleaning is of prime environmental importance. Most operators are using such drilling fluids in wells through these zones because of the borehole stability and drilling lubricity created by their use. Simon et al.'s research strongly indicated the use of oil-based muds throuoh North Sea chalk-zones because of the pore collapse and formation instability caused by water-based systems. Water-based systems developed and reported by Holder, however, contain lubricants and electrolyte inhibitors to control formation instability. Results presented show that universal application to horizontal-wellbore completions is possible. The areas of investigation include cement-slurry design, spacer design, effect of flow rate. optimal casing and hole-size determination, and centralization. Study of these five areas has resulted in a set of general recommendations to help create the maximum opportunity for achievement of a successful primary cement job in highly deviated or horizontal wellbores. Experimental Apparatus and Procedure The testing model used in this research is illustrated in Fig. 1. The facility in which this work was conducted has been accepted throughout the industry as one that adequately simulates downhole phenomena under controlled laboratory conditions. Since the early 1970's, many studies relating to cement-job optimization under various borehole conditions have been performed within this facility and reported in the literature. The apparatus consisted of the heating jacket, the casing/ wellbore simulation materials, and the wellhead assembly. The heating jacket was a 20-ft [6. 1 -m] section of 12-in. [30.5-cm] -ID casing with a flange connection on one end and sealed on the opposite end. This portion of the equipment was constructed so that circulation of hot water was possible, thus allowing for application of the desired bottomhole circulation temperatures (BHCT's) and bottomhole static temperatures (BHST's) to the wellbore during the testing sequence. Two casing sizes were studied in the work: 5- and 7-in. [12.7- and 17.8-cm] -OD casing. These pipes were cut into 18-ft [5.5-m] lengths for the study. As stated previously, the chalk formation being simulated was virtually impermeable. It was thus decided that 18-ft [5.5-m] sections of 8.5-in. [21.6-cm] -ID casing would provide adequate simulations of these conditions. These "formation" sections were equipped with an attached flange that matched the one on the heating jacket and were sealed on the opposite ends. The 5- or 7-in. [12.7- or 17.8-cm] casings to be cemented in the well were threaded so that attachment to the wellhead assembly was possible. The completed configuration (Fig. 1) allowed for the casing to be placed 6 in. [15.2 cm] from the bottom end of the wellbore. The wellhead consisted of a plug container and a valving arrangement that allowed circulation down the casing and up the wellbore, as well as pumping the top wiper plug at the end of the job simulation. The testing procedure entailed simulation of the entire life of the horizontal well from the initial drilling until the casing was cemented into place. The testing sequence involved the following steps. SPEDE P. 275^