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 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 (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 0.25
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
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 29-6
Unit 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.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
27 January 2004
- Revised manuscript received:
25 February 2005
- Manuscript approved:
2 April 2005
- Version of record:
15 June 2005
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