Cementing - A Systematic Approach

Abstract

Summary We developed a step-by-step cementing process design chart for Conoco Inc.'s operating divisions. It includes information on selecting cement and additives and the associated hardware for casing and liner cementing operations. Each cementing operation is designed to ensure the compatibility of hardware and cement slurry components. The chart is separated into two major categories - primary and liner cementing. These categories are separated further into design criteria for cementing each casing string in a well, conductor pipe through production string, and each type of liner that is set. Conoco has reduced primary cementing costs by as much as $35,000/well and has had fewer remedial cement jobs because of adherence to practices in the design chart. These cost savings can be attributed to proper cementing practices consolidated from many geographic regions that are included in this easy-to-use chart. For example, cement costs can be reduced by limiting the use of expensive synthetic cement additives in less critical pipe strings. Less expensive natural cement additives, which perform just as well, can be used to reduce the cost per cubic foot of cement slurry. Introduction Because of economic "belt-tightening" caused by depressed oil prices, all phases of drilling, completion, and production operations are undergoing cost reviews. When high-cost areas are identified, we must reduce costs without compromising design specifications. One such cost area is cement and cement additives that are used in oil, gas, and injection well cementing. During the drilling and completion phases of a well, all casing strings must be cemented to effect a hydraulic seal between the formation and pipe, to protect and to support the casing, and to isolate production zones. Failure to accomplish this can result in costly well repairs and may endanger equipment, lives, or the environment. For this reason, primary cementing operations must be carried out in such a way as to attain these cementing goals without resorting to expensive remedial squeeze cementing. The engineer must optimize primary cement slurry design and placement techniques. Casing and liner cement design generally is a joint effort among drilling, engineering, and cement service companies. The resulting design often is influenced by such factors as personal experiences, total cost or bids, profit margins, working-time safety factors, and equipment availability, rather than by prudent engineering based on available data. This method contributes to increased primary cementing costs, increased remedial costs, and sometimes, lost production. This paper describes how we have reduced cementing costs without compromising design specifications. Furthermore, in-house cementing schools and cementing studies are conducted to improve cementing practices and to reduce cementing costs. We will present case histories and the cementing process design chart that incorporates this philosophy in a systematic approach for cementing casing strings in a cost-effective manner. The Process Design Chart The process design chart in the Appendix will aid engineers and operations personnel in designing and executing successful cementing operations. While the chart is intended for general use, we realize that each well has individual characteristics that must be examined separately. Therefore, we do not specify how much additive is needed for a particular case, but we do indicate maximum amounts. When cementing recommendations exceed the maximum limits, the slurry design should be questioned and the reasons explained. We also consider casing hardware. The design of the chart generally proceeds from the well bottom to the top and reflects what is considered the best technology to obtain a uniform cement sheath around each casing string. For example, float equipment that has a spring-loaded dart valve generally is recommended over ball or flapper valves. This recommendation is based on calculations of the differential forces exerted on the seal during cement placement vs. the seal's resistance to failure. The chart does not necessarily recommend the use of the least expensive cement slurry, but it does inform the user about the available products and slurry design choices. The choice of what slurry and casing hardware to use will depend on well parameters and other design considerations the user feels are most important during the life of the well. The chart is designed so that the user can read from the top to the bottom of a casing string or liner column, and make decisions that are based on well requirements. By the end of the column, the user should be able to select an appropriate cementing procedure and corresponding cement slurry design, as well as the compatible casing hardware.