The nature and origin of borehole elongation recorded by the four‐arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in Auburn, New York. A preferred orientation N10°W‐S10°E, ±10° and a less prominant E‐W orientation of borehole elongation, was observed on two runs of the dipmeter. Comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm. Comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures, obtained from hydrofracturing tests and televiewer data show that the N10°W‐S10°E borehole elongations (1) are axisymmetric, (2) are aligned with the minimum horizontal stress Sh, and (3) are not associated with natural fractures intersecting the well. These elongations are interpreted as stress‐induced well bore breakouts. The E‐W elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts. This asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter. This study demonstrates that the dipmeter can be used to determine the orientation of Sh (by mapping breakouts), confirming the results of earlier less detailed studies, and provides a firm basis for mapping regional stress patterns using existing dipmeter data.
We use the low‐frequency reflected Stoneley‐wave mode to locate permeable fractures intersecting a borehole and to estimate their effective apertures. Assuming a model in which the average aperture of the fracture is roughly constant, theoretical work relates the magnitude of the Stoneley‐wave reflectivity to an effective fracture width, We treat both the case of a horizontal fracture and the case of a fracture crossing the borehole at an angle. Laboratory experiments verify the analytic solution for the case of a horizontal fracture. Full‐waveform array sonic data were also acquired in a wellbore with a long recording time (25.5 ms) in order to capture the late Stoneley‐wave arrivals. The data processing involves computation of the Stoneley‐wave reflectivity response using the measured direct and reflected Stoneley‐wave arrivals. A least‐squares fit to the arrival time of the reflected‐wave arrivals is used to estimate the locations of permeable fractures, and the effective width of the fractures is estimated by comparing the computed Stoneley‐wave reflectivity to the theoretical response from a parallel‐plate model. Test‐well results are consistent with a borehole televiewer analysis.
Summary This paper presents an evaluation of two fundamentally different stress models: an elastic model, which is based on linear transverse isotropic elasticity, and a failure model which is based on the concept that rocks are in an equilibrium state of shear failure. The models are first evaluated using physical parameters measured on core, pore pressure and in-situ stress data from the Gas Research Institute's pressure and in-situ stress data from the Gas Research Institute's Staged Field experiments in East Texas, It is shown that the elastic model and the failure model provide satisfactory predictions for most of the lithologies encountered. However, predictions for most of the lithologies encountered. However, the failure model is more accurate for predicting stress in soft shales. An example of stress predictions based on log derived elasticity parameters is also shown which gives stress estimations comparable to core based predictions. Introduction The influence of rock lithology on the state of stress has been the subject of an increasing number of studies during the last decades. These studies were initially applied to the determination of fracture pressure gradient to prevent fracturing while drilling then to containment of hydraulic fractures. More recently, the prediction and control of sanding and wellbore instability has increased the need for stress estimation. Initially, it was the lack of accurate stress measurements in rocks which led to the estimation of fracture gradient using a stress model. Various models were proposed but the elastic uniaxial strain model became quickly commonly used in the petroleum industry, certainly due to its simplicity. This model predicts the minimum stress from a knowledge of the overburden, pore pressure and Poisson's ratio. The Poisson's ratio used in the early predictions was either Poisson's ratio used in the early predictions was either constant or determined from previous fracture gradient measurements. In 1973, Anderson et al proposed to use log measurements to derive a better estimation of Poisson's ratio. They established a relationship between Poisson's ratio and formation shaliness as estimated using sonic compressional wave velocity and density. This relationship showed that the fracture gradient may be dependent on the lithology, if the assumptions of the elastic uniaxial strain model are valid. At the end of the 70's hydraulic fracturing dominated the subject of stress estimation. Analytical studies showed that fractures are easily contained by a stress contrast of about 3.5 MPa (500 psi) between the reservoir and the adjacent layers. Such a stress contrast, which was lithology dependent, was measured by Brechtel et al and Warpinski et al. Rosepiler used the elastic uniaxial strain model to predict stress containment and evaluated the success of the predict stress containment and evaluated the success of the predictions using acid fracturing data. Rosepiler found a predictions using acid fracturing data. Rosepiler found a good agreement between prediction and observation. More conclusive field validations could not be made without accurate stress, pore pressure and rock property measurements. A lack of physical basis was also the main criticism of stress models based on elasticity.
Many of today's well construction projects are technically and economically challenging. Examples include deepwater exploration wells in the Gulf of Mexico, offshore field development projects such as Hibernia, Newfoundland, Canada and onshore field development projects in tectonically active regions such as the Cusiana field in Colombia. Minimizing non-productive time associated with wellbore instability and unexpected pore pressure regimes reduces the risk of dangerous accidents and is required to complete the well on time and within budget. Minimizing non-productive time is a complex task that requires thorough pre-spud planning to identify drilling risks and geological hazards and to develop contingency plans for handling those risks. Building a mechanical earth model during the well planning phase and revising it in real time has proven to be extremely valuable in delivering complex wells safely while minimizing unplanned well construction costs. Monitoring and revising the model while drilling requires geomechanics expertise, teamwork, data management and excellent communications among service companies and their client. This paper defines a mechanical earth model, explains why it is important, how it is developed and how it is applied to well construction and field development. We will discuss sources of information and the multi-disciplinary team approach required to: generate, revise and maintain an earth model. Three examples of the application of the earth model concept are discussed. Introduction More of today's well construction and field development projects are both technically and economically challenging. Understanding the geomechanics of well construction is becoming increasing important in order to drill technically and economically challenging wells on budget. Wells with hostile pore pressure and fracture gradient profiles require a good pre-drill pore pressure and fracture gradient prediction in order to design a suitable casing program. A casing program designed on a profile significantly less hostile than that encountered may compromise the attainable TD of the well. The cost of materials and rig time spent running extra casing significantly adds to the cost of the well. The risk of taking kicks which can be both costly and dangerous can also be reduced by a more rigorous pre-drill pore pressure prediction coupled with real-time pore pressure analysis from LWD measurements. In the deepwater Gulf of Mexico there are examples of wells which require a good mechanical earth model (MEM) in order to be drilled at all. Despite decades of industry attention, wellbore instability is responsible for many costly stuck pipe incidents. Stuck pipe is responsible for lost BHAs and considerable NPT spent freeing pipe, performing additional wiper trips and hole cleaning. In cases where wellbore stability problems are severe, the economics of developing a field can become challenging, for example the Cusiana field in Colombia, S.A. Other fields where lesser wellbore stability problems may still challenge the field economics are found where the cost of drilling is very high, e.g. the Hibernia field offshore Canada and or fields in the North Sea.
Stress directions have been determined to depths of 4.5 km in eastern North America from borehole elongation measured by dipmeter calipers in 47 wells. The average maximum horizontal stress directions are eastern Canada, N54°E ± 7°; Appalachian Basin, N58°E ± 8°; and the Illinois Basin, N89°W ± 5°. Stress directions determined for the Illinois and Appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions. New results from eastern Canada and the Appalachian Basin show that stress directions are everywhere consistent with the Midcontinent stress province. These data suggest that the maximum horizontal stress does not rotate from the ENE Midcontinent trend to NW near the Atlantic coast. A directional analysis of hydraulic fractures, petal‐centerline fractures, and natural fractures from the Appalachian Basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures, centerline fractures, and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures. These observations indicate that borehole elongations provide reliable stress orientations.
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