The Use of Lightweight Cement Slurries and Downhole Chokes on Air-Drilled Wells
- Devin L. Brown (BJ Services Co.) | Thomas E. Ferg (ConocoPhillips)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2005
- Document Type
- Journal Paper
- 123 - 132
- 2005. Society of Petroleum Engineers
- 2.2.2 Perforating, 2 Well Completion, 3 Production and Well Operations, 1.14 Casing and Cementing, 1.11 Drilling Fluids and Materials, 4.6 Natural Gas, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.10 Drilling Equipment, 5.2 Reservoir Fluid Dynamics, 1.8 Formation Damage, 4.1.3 Dehydration, 1.14.3 Cement Formulation (Chemistry, Properties), 1.6 Drilling Operations
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The success of a primary cement job is often measured according to effectivezonal isolation, achieving the designed top of cement, and avoidance ofremedial cementing. In the San Juan and Rio Arriba counties of New Mexico, manyMesa Verde and Dakota wells are air-drilled because the Mesa Verde formation issubnormally pressured and naturally fractured.This combination makes theMesa Verde easy to hydraulically fracture during normal drilling and cementingoperations.Therefore, meeting the primary-cement-job success criteria isparticularly 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 ofthe hole, leading to hydraulic fracture initiation and loss of lift.Theuse of a downhole choke (DHC) can be employed to prevent these processes fromoccurring.
Because mud filter cake does not exist in an air-drilled hole, the leadwater spacer and cement must wet the casing and wellbore rocksurfaces.Loss of the lead water spacer, in conjunction with cement waterloss, can produce dehydration of the cement column, leading to an increase incement viscosity and equivalent circulating density.Careful control ofcement-free water and fluid loss are required to prevent this fromoccurring.Furthermore, placing cement across targeted completionintervals that have low fracture gradients requires the use of ahigh-compressive-strength, low-density cement that is competent and capable ofmaintaining zonal isolation during the completion and production phases of wellconstruction.
This paper will outline the evolution of the cementing practices used forthe Mesa Verde and Dakota wells drilled in the 29-6 Unit (T29N - R6W, NewMexico) from 1998 to 2002.It will illustrate how cementing changesenabled the primary cementing to be successfully completed in a single stageinstead of two, which has translated into cost savings for both the drillingand completion phases of a well. The impact of using a DHC for the pumping andplacement of these primary cement jobs will be illustrated and discussed.The function of the cement additives on slurry properties will also bediscussed as they pertain to these cement jobs.
The Mesa Verde and Dakota formations of the San Juan and Rio Arriba countiesof New Mexico are two of the primary completion targets for this area of theSan 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 illustratesthe Dakota and Mesa Verde formations, along with other productiveformations.The Dakota group is composed of a series of Cretaceouslenticular sandstones and shales.The nomenclature and designation of theindividual layers vary and are beyond the scope of this paper.The Dakotaformation is found at depths ranging between 7,000 and 8,000 ft, currentlymaking it the deepest productive interval in the San Juan basin.The MesaVerde group can be located at depths ranging between 4,500 and 6,500 ft and isalso composed of a series of Cretaceous formations.These consist of thePoint Lookout, Menefee, and Cliffhouse, from deepest to shallowest deposition,respectively.The Mesa Verde represents a group of subnormally pressuredreservoirs with original gradients having been measured at approximately 0.25psi/ft.However, continued pressure depletion of the Mesa Verde hasresulted in a reservoir gradient less than 0.15 psi/ft in most areas of the SanJuan basin.1
Because of the low pressure gradients associated with the Mesa Verdeformations, it has become a standard practice of many operators to useair-drilling techniques.2 These techniques use compressed air, natural gas, ornitrogen as the drilling fluid to dramatically reduce the hydrostatic-head andequivalent-circulating density. While this practice eliminates the concernsover lost circulation during drilling, it can pose significant challenges toachieving a successful primary cement job. A successful primary cement job isoften measured according to effective zonal isolation, achieving the designedtop of cement, and avoidance of remedial cementing.
This paper will discuss the challenges of achieving a successful primarycement job on an air-drilled well and how to design a lightweight cement slurryand casing configuration—inclusive of a DHC—to achieve a successful primarycement job.The experiences and evolution of a consecutive 6-year drillingprogram that produced 112 Mesa Verde and Mesa Verde/Dakota wells serves as thefoundation for support for these designs.However, for illustrationpurposes, the scope of this paper will be limited to the evolution of thesepractices 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 welllocations in the 29-6 Unit.It shows the study wells to be distributedover 23 of the 36 sections within the 29-6 Unit.The 29-6 Unit was chosenbecause it exhibits the lowest fracture gradients in the Mesa Verde and wasconsidered the worst-case scenario for achieving a successful primary cementjob on the production string of casing.
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