Feasibility Study of the Stability of Openhole Multilaterals, Cook Inlet, Alaska
- D. Moos (GMI) | M.D. Zoback (Stanford U.) | L. Bailey (Unocal Alaska)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2001
- Document Type
- Journal Paper
- 140 - 145
- 2001. Society of Petroleum Engineers
- 1.6 Drilling Operations, 3.2.5 Produced Sand / Solids Management and Control, 3 Production and Well Operations, 5.1.2 Faults and Fracture Characterisation, 5.2 Reservoir Fluid Dynamics, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.2.2 Geomechanics, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.14 Casing and Cementing, 2 Well Completion, 4.5 Offshore Facilities and Subsea Systems, 1.6.1 Drilling Operation Management, 1.10 Drilling Equipment, 5.6.1 Open hole/cased hole log analysis, 2.4.3 Sand/Solids Control, 1.2.3 Rock properties
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A study of in-situ stress, rock strength, and wellbore stability was initiated in the Hemlock sands of the McArthur River field, Cook Inlet, Alaska, to evaluate the potential of leaving the near-wellbore portions of multilaterals uncased. A northwest/southeast direction of maximum compression and a strike-slip faulting regime were predicted from analyses of leakoff test data and observations of failure (breakouts) in adjacent wells. Caliper, well-log, and core data indicated that cementation, and hence rock strength, is highly variable within the reservoir. Thus, it was decided to evaluate the stability of the lateral sections (i.e., the likelihood of wellbore failure during production) as a function of both stratigraphic position and well orientation. Laboratory rock-strength measurements were carried out on cores selected from target intervals in adjacent wells to provide sufficient precision to quantify the results. The results indicated that, while some reservoir intervals have high enough strengths to be left uncased when drilled in the most stable direction, these intervals are too thin to provide sufficient support at the point where the laterals leave the parent well. This justified the decision to case back the laterals to the parent well despite the cost.
Drilling problems and sand production frequently result from severe mechanical failure of the wellbore wall; therefore, they depend on the interplay between the magnitude and orientation of in-situ stresses, the rock strength, the wellbore and reservoir fluid pressures, and the orientation of the wellbore. Using a new suite of software tools developed to study wellbore stability in a wide variety of geologic environments, we can accurately predict optimally stable wellbore trajectories during drilling and production. The analysis is a two-step process. We determine the stress from observing the failures in existing wells; then we apply this knowledge to predict the stability of proposed wells while drilling and during production. In this paper, we illustrate this approach with an example in which we first determine the stress field using information from pre-existing wells, and then apply that information to predict the stability during drawdown of a series of multilaterals drilled from an inclined parent well.
The process of drilling a well results in the development of a stress concentration at the borehole wall.1,2 The stress concentration occurs because after drilling, the rock surrounding the hole must support the stress previously supported by the removed material. Because the magnitudes of the in-situ principal stresses are generally different (that is, the vertical stress, s, and the two horizontal stresses, sHmin and sH, are all unequal),3-5 the magnitude of the stress concentration varies markedly with azimuth around the well.1,6,7 Furthermore, the wellbore stress concentration depends on both the wellbore deviations, azimuth, and the magnitudes and orientations of the in-situ stresses.8-10
When the wellbore stress concentration exceeds the rock strength, the rock will fail. Failures can occur in compression, resulting in the development of wellbore breakouts2,11 or in tension, resulting in tensile wall fractures.12-15 These failures occur at azimuths that are a function of the stress magnitudes and of the orientations of the well and the principal stresses.15,16 Thus, wellbore failures detected in image logs or multiarm caliper logs can be used to determine the in-situ stress state.13,14,17-19 Once stress magnitudes and orientations have been determined from observations of wellbore failure, the stress state can be used to evaluate the stability of any well as a function of its trajectory.
In evaluating wellbore stability, it is important to note that wellbore failures can occur (and are quite common) without leading to loss of the well. For example, drilling-induced tensile wall fractures occur at the wall of the hole, but they do not extend away from the near-wellbore region or lead to circulation losses unless the mud weight exceeds the fracture gradient. Similarly, wellbore breakouts, which form over a discrete range of azimuths depending on the interplay between effective rock strength and the magnitude of the wellbore stress concentration, only jeopardize wellbore stability if the well loses arch support. In practice, this is likely to occur only if breakout widths exceed 90°. In some cases, wells will remain stable even if larger breakouts form. Breakout formation does, however, increase cuttings volume and makes cleaning the hole more difficult owing to the increase in effective hole size.
Geologic Background of the McArthur River Field.
The McArthur River field forms a north-northeast/south-southwest-trending anticline, typical of oil and gas reservoirs in the Cook Inlet, Alaska. The target-reservoir interval is the Oligocene-Age Hemlock formation, composed of interbedded, unconsolidated conglomerates, conglomeratic sandstones, and shales with a few minor coal seams. Within the McArthur River field, the Hemlock sands form a series of benches at depths of more than 8,000 ft.
The region of the Cook Inlet is cut by numerous northeast-trending faults. Some of these appear to have accommodated considerable reverse motion but have no evidence of recent activity. Others, such as the Castle Mountain fault that forms the northern boundary of the Cook Inlet basin, trend in more easterly directions and have been historically active as right-lateral strike-slip faults. This and other local faults accommodate residual relative motions associated with convergence of the Pacific plate beneath the overlying North American plate, which hosts the Cook Inlet fields.
Based on the recent tectonics summarized above, the region surrounding the Cook Inlet is clearly characterized by an active strike-slip/reverse faulting state. That is, the least principal stress sHmin is likely to be less than the vertical stress, and the greatest principal stress sH is likely to be significantly greater than the vertical stress. Furthermore, the maximum horizontal stress is likely to be oriented in a northwest/southeast direction, based on recent strike-slip faulting activity on faults such as the Castle Mountain fault. By itself, this information allows a qualitative assessment of the stability of wells drilled in this field. The next section begins the process of quantifying the stress state based on actual measurements.
Because the characteristics of wellbore failures depend on stress magnitudes, pore pressure, mud weight, and rock strength, it is possible to quantify the stress state with observations of wellbore breakouts and tensile wall fractures induced by drilling. This section presents the analysis methodologies used to define the stress state within the Cook Inlet McArthur River field.
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