Summary The success of a primary cement job is often measured according to effective zonal isolation, achieving the designed top of cement, and avoidance of remedial cementing. In the San Juan and Rio Arriba counties of New Mexico, many Mesa Verde and Dakota wells are air-drilled because the Mesa Verde formation is subnormally pressured and naturally fractured. This combination makes the Mesa Verde easy to hydraulically fracture during normal drilling and cementing operations. Therefore, meeting the primary-cement-job success criteria is particularly challenging. Cementing of casing within an air-drilled wellbore allows the slurry to freefall. Cement free fall can cause severe surge pressures at the bottom of the hole, leading to hydraulic fracture initiation and loss of lift. The use of a downhole choke (DHC) can be employed to prevent these processes from occurring. Because mud filter cake does not exist in an air-drilled hole, the lead water spacer and cement must wet the casing and wellbore rock surfaces. Loss of the lead water spacer, in conjunction with cement water loss, can produce dehydration of the cement column, leading to an increase in cement viscosity and equivalent circulating density. Careful control of cement-free water and fluid loss are required to prevent this from occurring. Furthermore, placing cement across targeted completion intervals that have low fracture gradients requires the use of a high-compressive-strength, low-density cement that is competent and capable of maintaining zonal isolation during the completion and production phases of well construction. This paper will outline the evolution of the cementing practices used for the Mesa Verde and Dakota wells drilled in the 29–6 Unit (T29N - R6W, New Mexico) from 1998 to 2002.It will illustrate how cementing changes enabled the primary cementing to be successfully completed in a single stage instead of two, which has translated into cost savings for both the drilling and completion phases of a well. The impact of using a DHC for the pumping and placement of these primary cement jobs will be illustrated and discussed. The function of the cement additives on slurry properties will also be discussed as they pertain to these cement jobs. Introduction The Mesa Verde and Dakota formations of the San Juan and Rio Arriba counties of New Mexico are two of the primary completion targets for this area of the San Juan basin. Fig. 1 shows a 500-square-mile view of the "Four Corners"area of the U.S.A., with the San Juan and Rio Arriba counties identified. Fig. 2 is a stratigraphic column of the San Juan basin. It illustrates the Dakota and Mesa Verde formations, along with other productive formations. The Dakota group is composed of a series of Cretaceous lenticular sandstones and shales. The nomenclature and designation of the individual layers vary and are beyond the scope of this paper. The Dakota formation is found at depths ranging between 7,000 and 8,000 ft, currently making it the deepest productive interval in the San Juan basin. The Mesa Verde group can be located at depths ranging between 4,500 and 6,500 ft and is also composed of a series of Cretaceous formations. These consist of the Point Lookout, Menefee, and Cliffhouse, from deepest to shallowest deposition, respectively. The Mesa Verde represents a group of subnormally pressured reservoirs with original gradients having been measured at approximately 0.25psi/ft. However, continued pressure depletion of the Mesa Verde has resulted in a reservoir gradient less than 0.15 psi/ft in most areas of the San Juan basin.1 Because of the low pressure gradients associated with the Mesa Verde formations, it has become a standard practice of many operators to use air-drilling techniques.2 These techniques use compressed air, natural gas, or nitrogen as the drilling fluid to dramatically reduce the hydrostatic-head and equivalent-circulating density. While this practice eliminates the concerns over lost circulation during drilling, it can pose significant challenges to achieving a successful primary cement job. A successful primary cement job is often measured according to effective zonal isolation, achieving the designed top of cement, and avoidance of remedial cementing. This paper will discuss the challenges of achieving a successful primary cement job on an air-drilled well and how to design a lightweight cement slurry and casing configuration—inclusive of a DHC—to achieve a successful primary cement job. The experiences and evolution of a consecutive 6-year drilling program that produced 112 Mesa Verde and Mesa Verde/Dakota wells serves as the foundation for support for these designs. However, for illustration purposes, the scope of this paper will be limited to the evolution of these practices on the 40 Mesa Verde and Mesa Verde/Dakota wells drilled in the 29-6Unit from 1998 through 2002.Fig. 3 is an aerial map of these well locations in the 29–6 Unit. It shows the study wells to be distributed over 23 of the 36 sections within the 29-6 Unit. The 29–6 Unit was chosen because it exhibits the lowest fracture gradients in the Mesa Verde and was considered the worst-case scenario for achieving a successful primary cement job on the production string of casing.
The success of a primary cement job is often measured according to effective zonal isolation, achieving the designed top of cement and avoidance of remedial cementing. In the San Juan and Rio Arriba Counties of New Mexico, many Mesa Verde and Dakota wells are air-drilled because the Mesa Verde formation is sub-normally pressured and is naturally fractured. This combination makes the Mesa Verde easy to hydraulically fracture during normal drilling and cementing operations. Therefore, meeting the primary cement job success criterion is particularly challenging. Cementing of casing within an air-drilled wellbore allows the slurry to free fall. Cement-free fall can cause severe surge pressures at the bottom of the hole, leading to hydraulic fracture initiation and loss of lift. The use of a downhole choke can be employed to prevent these processes from occurring. Because mud filter cake does not exist in an air-drilled hole, the lead water spacer and cement must wet the casing and wellbore rock surfaces. Loss of the lead water spacer in conjunction with cement water loss can produce dehydration of the cement column, leading to an increase in cement viscosity and equivalent circulating density. Careful control of cement-free water and fluid loss are required to prevent this from occurring. Furthermore, placing cement across targeted completion intervals which have low fracture gradients requires the use of a high-compressive strength, low-density cement which is competent and capable of maintaining zonal isolation during the completion and production phases of well construction. This paper will outline the evolution of the cementing practices used for the Mesa Verde and Dakota wells drilled in the 29–6 Unit (T29N - R6W, New Mexico) from 1998 to 2002. It will illustrate how cementing changes enabled the primary cementing to be successfully completed in a single stage instead of two, which has translated into cost savings for both the drilling and completion phases of a well. The impact of using a downhole choke for the pumping and placement of these primary cement jobs will be illustrated and discussed. The function of the cement additives on slurry properties will also be discussed as they pertain to these cement jobs. Introduction The Mesa Verde and Dakota formations of the San Juan and Rio Arriba counties of New Mexico are two of the primary completion targets for this area of the San Juan Basin. Figure 1 shows a 500 square-mile view of the "Four Corners" area of the United States, with the San Juan and Rio Arriba counties identified. Figure 2 is a stratigraphic column of the San Juan Basin. It illustrates the Dakota and Mesa Verde formations, along with other productive formations. The Dakota group is composed of a series of Cretaceous, lenticular sandstones and shales. The nomenclature and designation of the individual layers vary and are beyond the scope of this paper. The Dakota formation is found at depths ranging between 7,000 and 8,000 feet, currently making it the deepest productive interval in the San Juan Basin. The Mesa Verde group can be located at depths ranging between 4,500 and 6,500 feet and is also composed of a series of Cretaceous formations. These consist of the Point Lookout, Menefee, and Cliffhouse, from shallowest to deepest deposition, respectively. The Mesa Verde represents a group of subnormally pressured reservoirs with original gradients having been measured at approximately 0.25 psi/ft. However, continued pressure depletion of the Mesa Verde has resulted in a reservoir gradient less than 0.15 psi/ft in most areas of the San Juan Basin.1 Due to the low-pressure gradients associated with the Mesa Verde formations, it has become a standard practice of many operators to utilize air-drilling techniques. While this practice eliminates the concerns over lost circulation during drilling, it can pose significant challenges to achieving a successful primary cement job. A successful primary cement job is often measured according to effective zonal isolation, achieving the designed top of cement and avoidance of remedial cementing.
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