Filtrate Control - A Key in Successful Cementing Practices

Abstract

Control of filtrate loss from cementing slurry is as important to the successful cementing of a well as the control of slurry thickening time or the compressive-strength development. Examples of designed filtrate- loss controlled slurries are presented, illustrating the desirability of filtrate-loss control based on the characteristics of individual wells. Introduction In 1961, Beach et al. presented the results of squeeze-cementing perforations. The study was conducted over 4 years and was concerned with squeeze cementing on actual wells located primarily in South Texas. Their paper compares the squeeze practice of that time, which was one of attaining pressures equal to overburden pressure and "putting away" relatively large volumes of cement, with the new concept of low-pressure "hesitation," or the "walking" squeeze., The former method used normal slurries. The squeeze success ratio climbed from about 60 percent for the normal slurries to 85 percent for the filtrate-loss-controlled slurries. Perhaps the most spectacular success was a well in Perhaps the most spectacular success was a well in Yoakum County, Tex., where 230 ft of perforations was squeezed successfully with 100 sacks of cement in a single stage. Beach et al.'s paper is believed to be the first publication showing filtrate-loss control as a key to publication showing filtrate-loss control as a key to successful cementing. This work reversed the industry's thinking on squeeze-cementing techniques. From 1961 to 1967, low-filtrate-loss cement gained popularity for primary cementing, especially for liner popularity for primary cementing, especially for liner cementing where the annular clearance is small and high differential pressure is anticipated. Research studies during this period developed better additives and some unique placement techniques, such as long-life cement. Long-life cementing is basically a two-step operation: the cement is placed in the bottom of the hole, and then the casing is lowered into the cement. The method has been used successfully to cement full casing strings, liners, and multiple-string completions. Depending on the depth of the well and the capability of the rig, considerable time could elapse from the spotting of the cement until the pipe was in place. During this interval, the cement must be prevented from chemically setting and physically prevented from chemically setting and physically dehydrating. Low-filtrate-loss cements are a necessity for long-life cementing. Gas communication through cemented intervals was recognized at about this same time. The problem, was first noted in gas storage wells and later in gas producing wells. Laboratory model studies were conducted to determine the source of the problem . Of the variables, or factors, studied in the models, cement dehydration was found to be the second most important factor contributing to gas leakage into the wellbore. Cement-slurry density took first place. It should be emphasized that all factors contributing to gas migration into the wellbore in some manner reduced the hydrostatic pressure in the borehole to less than the formation pressure. Later work showed displacement techniques also aided in preventing gas leakage. Application of low-filtrate-loss cements and pipe movement coupled with turbulent-flow cementing pipe movement coupled with turbulent-flow cementing rates have helped in overcoming some of the gas leakage problems. problems. Premature dehydration of the cement slurry caused by lack of fluid-loss control may be the primary cause of gas communication. JPT P. 951