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Emerging small-particle-size cement technology has been applied successfully to one of the oil field's most frustrating problems, repairing tight casing leaks. Because of these leaks, a continuous injection rate cannot be established, yet test pressure leaks off 300 to 500 psi in 10 to 20 minutes. Confirmed through experimental data and more than 100 field operations, a new approach offers exceptionally high success rates for these troublesome leaks. This paper presents a detailed introduction to these new cement systems, along with comparative studies of small-particle-size cement vs. standard oilfield cements. This paper also discusses repair methods, several case histories, and field jobs performed to date.
Once a well has been drilled and all casings have been cemented in place, it is difficult to forecast accurately the problems that will occur throughout the productive life of the well. One prediction, however, is certain: if a well remains in service long enough, the casing will leak.
When a casing leaks, regardless of the cause, a competent repair usually is required to ensure wellbore integrity. Cementing the leak with squeeze techniques often becomes the method of repair. While many casing leaks have been repaired successfully in this manner, the casing leak sometimes is so restrictive to fluid injection that it does not lend itself to repair by conventional cementing methods. This type of casing leak may be too small to allow penetration of cement particles and, in many cases, requires several cementing operations for successful repair. Even worse, many of these tight leaks cannot be repaired and result in well abandonment.
In response to this problem, small-particle-size cement was developed. This development is based on the simple premise that minimizing particle sizes in a cement slurry will maximize the slurry's penetrating capabilities. In this manner, "tight" leaks previously inaccessible to conventional cement slurries have now been repaired by small-particle-size cements that can enter the leak.
Permian Basin operators who need to seal casing leaks to pass Permian Basin operators who need to seal casing leaks to pass Texas Railroad Commission mechanical integrity tests have successfully applied small particle-sized cements, which penetrate casing cracks and very small channels to effect a penetrate casing cracks and very small channels to effect a seal.
Small casing leaks plague injection operations when integrity testing results in pressure bleedoff, which indicates presence of leaks. A batch-mixed volume of 25 to 75 sacks of fine particle cement is the typical application used to seal these particle cement is the typical application used to seal these leaks. There are some critical steps that must be taken to help ensure job success; these are discussed. The paper also presents a discussion of small particle-size cements, including presents a discussion of small particle-size cements, including accounts of laboratory testing. Case histories describing techniques of casing leak repair with small particle cements are recounted.
The Permian Basin area of West Texas is known for many waterfloods, H2S problems, and weak formations. The Permian Basin has a long drilling history, and it therefore Permian Basin has a long drilling history, and it therefore contains numerous old wells (up to 50 years old), the casings of which are subject to corrosion degradation. These characteristics make Permian Basin wells subject to casing leaks and cause wells to fail casing integrity tests established by the Texas Railroad Commission for injection wells.
Most casing leaks in the Permian Basin that have been repaired with small particle-size cement have ranged from 2000 to 12,000 ft in depth. Most of these have been shallow repairs, 5000 ft and above with some as shallow as 100 ft.
Squeeze cementing with small particle-size cement has proved to be an excellent way to seal casing leaks without requiring perforating. Small particle-size cement squeeze jobs do not perforating. Small particle-size cement squeeze jobs do not just patch the damaged area but penetrate to create a seal, Because the need to perforate is eliminated, many small particle-size cement squeeze jobs are less costly than squeeze particle-size cement squeeze jobs are less costly than squeeze jobs involving standard cement.
Many successful squeeze sealing jobs using small particle-size cement have been performed in the Permian Basin, The likelihood of a successful small particle-size cement squeeze job is enhanced by incorporating certain preparatory and procedural steps advocated within this paper. procedural steps advocated within this paper. Small Particle-Size Cements
There are two types of small particle-size cement available; within this paper they are referred to as "fine" and "ultrafine."
Both fine and ultrafine cements are very finely ground cements with average particle sizes much smaller than standard API cement. The small particle size of these cements make them well suited for all squeeze jobs, especially casing and collar leaks in which cement must penetrate very narrow or "tight" areas. Table 1 compares physical properties of fine, ultrafine, and standard cements. properties of fine, ultrafine, and standard cements. Fine cement is a very finely ground cement with an average particle size of 8 microns and a maximum particle size of 15 particle size of 8 microns and a maximum particle size of 15 microns. Fine cement consists of 20 to 30% finely ground cement and 70 to 80% slag material.
Ultrafine cement consists of 100% very finely ground Portland cement and has an average particle size of 4 microns. Its extreme fineness makes it very reactive but provides it with excellent penetration capabilities.
The benefits of small particle-size cements have been proven in hundreds of jobs which required sealing a casing collar leak or other very narrow area. In these cases, traditional API cements bridge on the affected area, but small particle-size cements penetrate to provide a much more complete seal without requiring perforating. Other applications of small particle-size cements include penetration of gravel packs, particle-size cements include penetration of gravel packs, sealing highly permeable zones, stopping unwanted water or gas production in behind-pipe channels, and squeezing small channels.
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The requirements for zonal isolation in Conoco's Hutton field have greatly increased as production from the field continues. High differential pressures approaching 1500 psi between zones which are less than 15' apart have made obtaining effective isolation of paramount importance. Problems with zonal communication behind the Problems with zonal communication behind the production casing were experienced due to an production casing were experienced due to an incomplete cement sheath. High pressure water zones flowed through cement channels to a low pressured oil zone and reduced oil production. In addition, water injection production. In addition, water injection did not enter the desired zone.
A review of the methods used for primary cementing was completed and recommendations were made to improve the quality of the cement job. These recommendations were then implemented on subsequent wells. These recommendations included practices for conditioning the hole for the casing, designing the cement slurry and spacer system, and running and cementing the casing.
A high priority was placed on preparation of the hole for casing. Drilling practices were addressed such as maximum preferred hole angle, circulation rates, properties of the drilling fluid and making a conditioning trip after logging.
The cement slurry design was addressed in concert with the mud and spacer systems. A new test was devised to design a spacer system suitable for water wetting and mud filter cake removal. Rheological, settling and thickening properties of the slurry were addressed.
Casing running and cementing practices were reviewed. Sandblasting the casing, casing centralization, casing running speeds and slurry volumes were engineered. Fluid properties and circulation rates were properties and circulation rates were optimized prior to cementing, as well as the use of multiple wiper plugs for separation of the mud, spacers and cement. Selection of cement mixing equipment and onsite supervision were also addressed.
As a result of this work, substantial improvements in primary cementing have resulted in near perfect cement bonds and elimination of zonal communication in the Hutton field.
The Hutton Field was discovered in 1973 in the UK sector of the North Sea (Figure I). The field was developed using the world's first commercially operated tension leg platform (TLP). Initially 10 wells were platform (TLP). Initially 10 wells were predrilled through a 32 slot template. predrilled through a 32 slot template. Since installing the TLP, 18 wells have been drilled and several side tracks have been completed.
The Hutton field geological structure consists of three fault blocks, a structure map is shown in Figure 2. The reservoir contains four sand members. A typical cross section of the reservoir is shown in Figure 3. The sand members vary in thickness from 35' to 65' (10.7 to 19.8 m). Due to the nature of the faulting, sand deposition and field development, recently drilled wells have encountered high differential pressures between oil and water bearing zones which are relatively close together. A pressure differential of 400 - 800 psi (2758 - 5516 KPa) between the Massive Sand and Basal/Mica/Middle Shaly Sands are a common occurrence in the field. Figure 4 shows the formation pressure data from a recently drilled well. On another well, a 1500 psi (10,342 KPa) differential pressure was evident between 2 zones which were less than 15' (4.6 m) apart. Obtaining zonal isolation between these sand members is the most important part of drilling in the Hutton field.
Problems have been encountered on wells Problems have been encountered on wells where an incomplete cement sheath has allowed a high pressure water zone to flow behind pipe and reduce oil production from a low pressure oil zone. A successful primary cement job is essential to ensure that when the well is perforated only the fluids from the intended zone are produced. A similar requirement exists for an injection well, where only the target zones should receive fluid.
Until recently, wells in the northern Sacramento Valley had often been troubled by interzonal communication, as evidenced by poor bond logs necessitating frequent remedial cement work. Cement designs commonly used in this area proved ineffective in solving this problem. Analysis of the cementing failures in this gas field led to the realization that a total reevaluation of the cementing program for this field was required to determine the nature of the poor cement job results obtained.
An effective cementing program was eventually developed based upon findings published in research papers on cementing practices, and on field experience already obtained in this particular area. This cementing program focused on cementing practices including displacement mechanics, slurry design, and physical elements such as cement column length. Through a thoroughly engineered approach to well evaluation and cement job design, a consistent methodology for cementing practices was developed. Two key elements of the successful cement design included use of controlled fluid loss cement slurries, and short cement column lengths.
Though this paper presents a summary of field experiences specific to the conditions prevalent in the northern Sacramento basin, the methodology should be applicable to any given set of conditions either on a field-wide or individual well basis. An examination of this analytical, problem solving cementing design technique will show operators and service companies a method to help minimize job design flaws, and to optimize cementing results.
Standardization of cementing practices in the northern Sacramento valley utilizing consistent procedures and methodology established as a result of detailed research investigations has led to a dramatic improvement in cementing results. Previous cementing practices had given a success ratio of approximately 65%, as judged by poor cement bond logs and remedial cement work needed to "fix deficient primary cement jobs. Since the development of the "optimized" cementing program, the success ratio has climbed to approximately 95%. Features of the optimized cement design program include implementation of recommendations outlined in published results of previous investigations evaluating the influence of various cementing practices and their effectiveness, and the presentation of previous cementing practices in the northern Sacramento valley, evaluation of those practices and their effectiveness, and the establishment of new guidelines for cementing standards will show how the application of a consistent methodology has led to improved cementing results and established am analytic approach to optimize cementing design and performance.
The Sacramento basin is an almost exclusively gas producing province which has been and continues to be California's most active exploratory area. The northern Sacramento basin is comprised of many small fields. Production is primarily from the Campanian Forbes, a 2,000 to 3,000 ft thick deposit containing numerous interbedded, thin, and discontinuous sand bodies deposited in low-relief channels and/or depositional lobes.
Setting openhole cement plugs has historically been a difficult task. Often, several attempts are required to set a cement plug before one is obtained that has sufficient strength for sidetrack and is at the intended depth. It is our opinion that unstable flow behavior, resulting from the heavy weight of an unset cement plug resting on top of a lightweight mud, is one of the main causes of plug failure. The common practice of using open-ended drillpipe or tubing to spot the plug is also a major contributor to unsuccessful plug setting. This paper presents a placement technique and materials design that have resulted in a successful plug on the first attempt in many field cases where previous plug setting was difficult. Several case histories are presented. To examine the instability problem associated with cement plugs, an experimental investigation was conducted. This investigation consisted of setting cement plugs of known densities and rheological properties on top of drilling muds having known properties. The experiments were conducted using lucite pipe to simulate hole or casing and using copper tubing as the drillpipe. In each test, the hole was initially filled with drilling mud. Then, cement was displaced through the simulated drillpipe to displace drilling mud up the annulus. The ability of the cement plug to remain where it was placed was observed visually and correlated with other experimental observations. This paper includes the results of the experimental paper includes the results of the experimental investigation. The main conclusion of this investigation is that the stability and quality of a cement plug for sidetracking or any other purpose can be greatly improved through the use of our new placement technique.
Early in 1980, a research project was directed toward determining the possible causes of repeated failures in setting cement plugs, particularly sidetrack plugs, as reported for some operating areas. Those reported failures seemed more common when plugs were placed in wells with drilling muds weighing less than 10 lbm/gal [1198 kg/m ]. As part of the investigation, a detailed literature study was initiated. More than 150 articles dealing with many aspects of setting cement plugs were screened. For detailed study, a smaller group of papers that appeared to have the most relevant information was selected. Those articles and a few others are given in the General References at the end of this paper. The chosen group of articles gave a consensus concerning techniques and slurry design considerations needed for successfully setting cement plugs (a list of those considerations that we concur with is given in Appendix A). After investigating several of the reported failures, it was concluded, with some exceptions, that those industry-recognized techniques given in Appendix A were being followed. However, plug failures were still occurring. In a previous paper, Beirute investigated the behavior of a liquid cement plug once it is left alone in the well. By means of laboratory experiments and a mathematical model, he showed that heavy cement slurries placed on top of lightweight muds form unstable interfaces which because of the adverse gravitational forces may cause the fluids to flow, contaminating the plug or causing it to move downward and, therefore, contributing to the plug job failure. His observations when running his experiments correlate quite well with field descriptions of reported cement plug failures. Since those observations are considered quite significant, they are given in Appendix B. A series of experiments were designed to investigate the phenomenon of cement plug stability in the wellbore. The idea was to determine whether this was a major contributor to cement plug failures, particularly when setting plugs by the balanced plug method, without using a bridge plug or any other form of support "table" below the plug. Also, if plug instability was found to be a serious cause, of failure, it was the intention of the investigation to discover ways to control the problem.
The experiments were conducted in a simulated wellbore constructed out of clear lucite tubes approximately 18 in. [46 cm] long and 1.75 in. [4.4 cm] ID as shown in Fig. 1. Some tests were run in longer tubes (approximately 3 ft [0.9 m] long). To simulate the drillpipe, 0.5-in. [1.3-cm] copper tubing was used. The drilling mud and cement slurries that were used in the experiments were mixed as closely as possible to those used in the field. A low-solids, nondispersed (LSND) drilling mud that weighed 9.0 Ibm/gal [1078 kg/m ] was used in the experiments. This type of mud is used extensively and was chosen since it represented the greatest density difference between the cement slurry and the drilling mud that would be encountered in the majority of situations where cement plugs would be set. A list of the fluids used in the experiments is given in Table 1. The initial experiments were conducted to simulate placement techniques as closely as possible. placement techniques as closely as possible. JPT