Wellbore Orientation Research Articles

Influence of Intersection Angle at Single-Well Flow Tests in Fracture-Wellbore Systems

Abstract The intersection angle between a hydraulically effective fracture and an active wellbore influences the response observed in single-well tests. An experimental study was launched to investigate the ability of a semi-analytical model proposed to account for this influence. The setup designed for this purpose includes three distinct fracture-wellbore system models with 90°, 20°, and 10° intersection angles, each assembled into a steel box frame to provide a constant fracture aperture during testing. The laboratory program involved testing each of these models for three different fracture apertures under steady, constant-flux, injection, and pumping conditions. The overall experimental setup successfully simulated the conceptual testing environment in which the mathematical model is expected to reproduce. Analysis of the test results confirmed the effectiveness of the conceptual model in refining the predictions of single-well tests. Important applications include the determination of anisotropic rock-mass permeability in a single, arbitrarily oriented wellbore by isolating fractures of different sets and of optimum wellbore orientation and spacing for maximum efficiency of producing wells.

Characterization of Natural Fractures in the Frontier Formation, Green River Basin, Wyoming; Application to Reservoir Simulation of Horizontal Wells: ABSTRACT

Significant gas reserves are present in the low-permeability sandstones of the Frontier Formation in the Green River Basin, Wyoming. Successful exploitation of these reservoirs through horizontal drilling requires understanding the orientations and distributions of natural fractures. Outcrop studies, core analyses, and welltest data have been integrated to characterize these reservoirs and implement a realistic simulation of horizontal well productivity. Detailed maps of fracture systems have been constructed from outcrop pavements, of the Frontier sandstones at the edge of the basin. They show relatively uniform distributions of vertical regional extension fractures and that bed thickness is the dominant geologic parameter controlling fracture behavior. Spatial frequency distribution of features has been derived from statistical analysis of fracture orientation, spacings, lengths, and interconnections and correlated to bed thickness. Fracture spacing and length decrease and fracture interconnectivity increases with decreasing bed thickness. A natural fractured reservoir model has been developed using this fracture relationship. This model incorporates fracture and matrix permeability determined in laboratory tests conducted at in-situ reservoir conditions. The model shows that the relationship between bed thickness and fracture frequency distribution and the contrast between matrix and fracture permeability determines the reservoir`s effective transmissibility. Optimum productivity of horizontal wells has been calculatedmore » as a function of wellbore orientation and length relative to bed thickness to guide field development. Wellbore and welltest data provide validations for the geologic predictions of fracture characteristics and for simulation predictions of reservoir response.« less

A practical method for estimating the orientation of eolian dune sands from deviated wells; an example from the Rotliegendes Group of the southern North Sea

The identification of different sedimentary structures in eolian dune deposits (particularly those resulting from grain flow) allows estimation of the depositional dip angles of some bedding planes. By combining this dip-angle estimation with the overall dip of the formation (derived from seismic data and well correlation) and the wellbore orientation of a deviated well, two possible bedding-plane azimuths for the grain-flow deposits can usually be defined. By choosing wells deviated in a suitable direction and by assuming that major dune slip faces have a 180 degrees range in possible dip azimuth centered around the regional formative wind direction, the cores can be oriented when the regional formative wind direction is known. Detailed data on bedding dip may thus be extracted from conventional unoriented cores in suitably deviated wells. This method can be used in conjunction with or in the absence of other core-orientation techniques and has been applied successfully in parts of the Rotliegendes Group of the southern North Sea. However, it may also be applied to any cored eolian dune deposits where a single regional wind direction has previously been determined and the well has a suitable deviation direction.

THE STRESS FIELD OF THE NORTH WEST SHELF AND WELLBORE STABILITY

Boreholes drilled in the search for hydrocarbons in the Barrow-Dampier Sub-Basin (North West Shelf, Australia) commonly exhibit an elliptical cross-section believed to be due to stress-induced wellbore failure known as borehole breakout. The azimuths of the long axes of 138 discrete breakouts identified in nine different wells in the Barrow-Dampier show a consistent 010°−030°N trend implying that maximum horizontal compressive stress is oriented 100°−12G°N.The orientation of horizontal stress determined in this study (and that from the Timor Sea area which is rotated some 50°−60° with respect to the Barrow-Dampier) is consistent with that derived from theoretical modelling of the stress within the Indo-Australian plate based on the plate tectonic forces acting on its boundaries. The rotation of the horizontal stress orientations along the North West Shelf, between the Barrow-Dampier and the Timor Sea, is a reflection of the present-day complex plate convergence system at the north-eastern boundary of the Indo-Australian Plate.Vertical stress magnitudes, Sv, in the Barrow-Dampier were determined from density and sonic log data. Minimum and maximum horizontal stress magnitudes, Shmin and Shmax, were determined from mini-hydraulic fracture (or modified leak-off) test results. These data suggest that the fault condition of the Wanaea/Cossack area is on the boundary between normal faulting (extension) and strike-slip, i.e. Sv ≈ Shmax > Shmin. However, in other parts of the Barrow-Dampier the evidence suggests a strike-slip fault condition, i.e. Shmax > Sv > Shmin.Given the orientation of the stress field and the fault condition, inferences can be drawn regarding the stability of horizontal wells. Furthermore, experience from vertical wells can be utilized to determine the upper and lower bounds to the mud-weight envelope as functions of deviation and wellbore orientation. Since a horizontal well will see Sv and a horizontal stress, stress anisotropy around a wellbore in the Wanaea/Cossack area (and hence wellbore instability) will be minimized by drilling in the Shmin direction i.e. 010°–030°N.

A Practical Method for Estimating the Orientation of Eolian Dune Sands from Deviated Wells: An Example from the Rotliegendes Group of the Southern North Sea: RESEARCH METHOD PAPER

ABSTRACT The identification of different sedimentary structures in eolian dune deposits (particularly those resulting from grain flow) allows estimation of the depositional dip angles of some bedding planes. By combining this dip-angle estimation with the overall dip of the formation (derived from seismic data and well correlation) and the wellbore orientation of a deviated well, two possible bedding-plane azimuths for the grain-flow deposits can usually be defined. By choosing wells deviated in a suitable direction and by assuming that major dune slip faces have a 180° range in possible dip azimuth centered around the regional formative wind direction, the cores can be oriented when the regional formative wind direction is known. Detailed data on bedding dip may thus be extracted from con entional unoriented cores in suitably deviated wells. This method can be used in conjunction with or in the absence of other core-orientation techniques and has been applied successfully in parts of the Rotliegendes Group of the southern North Sea. However, it may also be applied to any cored eolian dune deposits where a single regional wind direction has previously been determined and the well has a suitable deviation direction.

An On-Site Method For Optimizing Oil Recovery During Drilling Of Deviated And Off-Target Wells At Norman Wells

Abstract The thick oil zone and the close well spacing in the anisotropically fractured Norman Wells reservoir make it important to locate and orient wells properly. Nearly eighty per cent of the wells drilled in Norman Wells wilt be directionally drilled. A method has been developed for the preplanning and on-site optimization of directional drilling in a closely-spaced jive-spot waterflood pattern. This technique was the key to evlaluating the economic benefits of drilling S-bend well profiles which provide a near vertical trajectory through the reservoir. S-bend drilling profiles have been used successfully on wells up to 60 degrees, while higher angle wells (to 75 degrees) have been drilled using the more conventional build-and-hold profiles without serious drilling problems. The technique is used to optimize the wellbore configuration within the reservoir and to justify drilling expense for directional correction runs and plugback /sidetrack operations. The theory has been incorporated into a hand-held calculator. The program allows an on-site drilling engineer to calculate combined effects off-target drilling and wellbore orientation for both the pre-drill plan and while drilling projected trajectories through the 150 m thick carbonate reef. This enables the driller to weigh the benefits of alternate drilling trajectories against the cost of deviating from the projected wellbore path. As a result, success has been achieved in ensuring better waterflood conformance by providing the quantitative directional evaluation tool required to optimize well location and trajectory. Introduction The Norman Wells oil reservoir is currently being developed by Esso Resources Canada. More than half of the planned 162 wells have been drilled to date. Waterflooding of this thick naturally fractured reservoir will utilize an elongated five-spot waterflood pattern with 6.2 ha spacing. Start of the waterflood systems is scheduled for mid-1984 with planned start-up of the production facilities in 1985 following completion and tie-in of the Norman Wells pipeline to the Rainbow-Zama pipeline system some 870 km away. Figure 1 shows the location or Norman Wells on the Mackenzie River and the Norman Wells pipeline to Rainbow Zama. Although Norman Wells was discovered in 1920 and has been producing continuously since the early 1940s(1), the remote location of the field and the difficulty of accessing the 60% of the reservoir under the Mackenzie River have precluded development to this time. Prior to the project, development of the Norman Wells field was limited to three areas; Bear Island, Goose Island and the mainland. Until the mainland waterflood was started up in 1980, depletion of the pool had been by primary depletion. To date, oil production has totalled four million m3, approximately 4% of initial oil-in-place. Under waterflooding it is expected that 42% of initial oil-in-place will be recovered. Development of the Norman Wells reservoir is planned from the three existing areas plus six artificial islands constructed in the Mackenzie river, or the 162 wells planned for development, 78% will be directional wells.

Waterflood Oil Recovery in Fractured Reservoirs With Directionally Drilled Wells

Abstract A technique has been developed to estimate the potential effects of directionally drilled wellbore orientation on pattern waterflood oil recovery in anisotropically fractured reservoirs. The technique attempts to quantify the tradeoff between drilling directional wells either more vertically or better aligned with the major fracture orientation in situations where simple vertical wells are not possible. Among the incentives to deplete some reservoirs with directionally drilled wells are the ability to access reserves located beneath large bodies of water from shore or island structures and the economy of centralized surface facilities. The orientation of these directionally drilled wells in anisotropically fractured oil reservoirs may have a significant impact on recovery efficiency. The described method involves combining the directional permeability characteristics of the reservoir caused by fractures, drilling accuracy, and the proposed wellbore orientation to estimate the volume of reservoir that may be affected by a nonvertical well. The distribution of fractures in the reservoir, average fracture length, and effective vertical permeability are noted as being major factors influencing the effect of directionally drilled wells on oil recovery. When applied to the Norman Wells oil field (N.W. Ter., Canada), it was possible to identify elongated target areas within which any directionally drilled well is expected to have similar oil recovery.