Summary
A significant portion of the US gas resources is located in
low-permeability, bypassed pay zones, within multilayered sandstone-shale
sequences. Acquiring these resources leads to operational and design
difficulties in stimulation, particularly if the target zone is bounded above
and below by existing producing zones. The objective of this work was to
evaluate the impact of adjacent, existing producing zones on the stimulation
design and therefore production performance of the bypassed payzone.
To investigate this problem, a 3D planar, hydraulic fracture propagation
model was constructed and superimposed on a 3D flow model. The physical model
comprises three layers, the top and bottom representing previously stimulated
and producing layers, and the middle layer the target or bypassed layer. The
impact of lithology, fracture length, and total stress variations over time on
the fracture conductivity, fracture efficiency, and average reservoir pressure
were investigated.
Evidence of pressure depletion of the target layer was observed caused by
production of the upper and lower layers. The degree of depletion is dependent
on the fracture length and lithology of all of the layers. That is, the ability
to propagate a fracture in the target layer was a strong function of the shale
content and to a lesser extent, on the hydraulic fracture length of the
bounding layers. Increased shale content in the target as well as the bounding
layers resulted in a decrease in fracture conductivity of the target layer.
However, an increase in fracture length did not necessarily result in a
decrease in fracture conductivity of the target layer.
The study includes examples of stimulating the Menefee formation in the San
Juan basin.
Introduction
The purpose of this paper is to analyze the overall behavior of a multilayer
formation, paying special attention to the effect of the current productive
zones on the production and stimulation of the bounded, middle layer. To
evaluate the general lithologic sequences, it is necessary to properly define
both the design goals and the design variables that affect the overall process.
Because both production and stimulation are considered, concepts on hydraulic
fracturing (i.e., elasticity theory, rheology, continuity, and fracture
mechanics) and fluid flow (i.e, continuity, flow rules, and state equations)
must be incorporated for the design goals to be fully coupled. Also, because
the common element in both hydraulic fracturing and fluid flow theories is the
sensitivity of the medium to stress, the incorporation of nonisotropic
permeability tensors and stress maps is necessary to properly describe the
system.
The model considers a formation consisting of three layers, of which the top
and bottom have been stimulated and producing for a period of time. After
several years of production, the second (middle) layer is stimulated, and the
entire system (three layers) produce to the wellbore. The proportion of propped
target length among layers has been set to one (i.e., 750 feet [ft]) or two
(i.e., 1,500 ft), and therefore in an X:1-2-2 sequence, the propped length for
the first layer is 750 ft., whereas it is 1,500 ft. for the second and third
layers, respectively.
The impact of four different types of rocks are investigated as shown by
Fig. 1. These impacts, along with the number of layers that compose the system,
lead to the selection of only eight possible lithologic sequences, on the basis
of the following restrictions:
- The first and third layer is never either "sandy shale" or
shale.
- Two contiguous layers cannot share the same type of rock.
Fig. 2 depicts the considered sequences.
From the previously mentioned considerations, a methodology for the
development of a fully coupled model is proposed, and the corresponding
lithologic sequences are evaluated and optimized.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 February 2007
- Meeting paper published:
16 April 2007
- Revised manuscript received:
3 June 2008
- Manuscript approved:
19 June 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009
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