A fresh look at Wellbore Stability Analysis to Sustainable Development of Natural Resources: Issues and Opportunities

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1 A fresh look at Wellbore Stability Analysis to Sustainable Development of Natural Resources: Issues and Opportunities Dr.Parag Diwan, Dr.B.P.Pandey, Dharmendra Kumar Gupta*, Suresh Ayyappan Department of Petroleum Engineering, University of Petroleum & Energy Studies Dehradun, Uttarakhand INDIA. * Corresponding Author dkgupta@ddn.upes.ac.in Phone: Abstract: To meet the energy needs and demands globally, exploitation work of natural resources is essential in this present scenario. So in the exploitation work of natural resources like hydrocarbons, care should be taken in safeguarding the environment and its associated ecological problems so that all petroleum related activities were done after meeting all the necessary environmental legislations and clearances. Meeting all necessary environmental legislations and clearances to carry out any petroleum related activities doesn t guarantee in safeguarding the environmental concerns and its associated ecology because of practical difficulties in predicting the behavior of the mother earth as it is a highly complex subsurface behavior. Because of the energy needs and demands today, we have to meet the needs of the present without compromising the ability of future generations to meet their own needs. Hence the concept of Sustainable Development was introduced in all the exploitation work of natural resources. Sustainable development is a pattern of resource use that aims to meet human needs while preserving the environment so that these needs can be met not only in the present, but also for generations to come. Wellbore stability issues have been estimated to cost the industry US$8 billion every year. Key Words: Wellbore Stability, Stress Analyses, Sustainable Development, Natural Resources Introduction Use of appropriate technology is very essential in the concept of sustainable development. Today appropriate technologies were used by all major oil companies to effectively carry out their exploitation work of hydrocarbon resources. In these exploitation works, Petroleum Geomechanics field is the least addressed area. International Journal of Science Technology & Management Page 55

2 Nowadays the use of inclined and horizontal wells in the exploitation of natural resources was increased at a higher rate and so the need of wellbore stability while drilling arises. This is particularly true for long reach, highly deviated and horizontal wells where the cost of downtime is very high. The development of a comprehensive geotechnical model of a reservoir (and overlaying formations) provides a basis for addressing a wide range of problems that are encountered during the life-cycle of a hydrocarbon reservoir. These include questions that arise during the exploration and assessment phase of reservoir development such as the prediction of pore pressure, hydrocarbon column heights and fault seal (or leakage) potential; during the development phase where engineers seek to optimize wellbore stability through determination of optimal well trajectories, casing set points and mud weights and geologists attempt to predict permeability anisotropy in fractured reservoirs; throughout the production phase of the reservoir that requires selection of optimal completion methodologies, the prediction of changes in reservoir performance during depletion and assessment of techniques, such as repeated hydraulic fracturing, to optimize total recovery. Importance of Stress: The important parameter in the geomechanical model is knowledge of the current state of stress. Wellbore failure occurs because the stress concentrated around the circumference of a well exceeds the strength of a rock. Earth Stress Compressive stress exists everywhere at depth in the earth. Stress magnitudes depend on depth, pore pressure and active geologic processes that act at a variety of different spatial and temporal scales. There are three fundamental characteristics about the stress field that are of first-order importance (i) Knowledge of stress at depth is of fundamental importance for addressing a wide range of practical problems in geomechanics within oil, gas and geothermal reservoirs and in the overlaying formations. International Journal of Science Technology & Management Page 56

3 (ii) (iii) The in situ stress field at depth is remarkably coherent over a variety of scales. These scales become self-evident as data from various sources are analyzed and synthesized. It is relatively straightforward to measure, estimate or constrain stress magnitudes at depth using techniques that are practical to implement in oil, gas and geothermal reservoirs. State of Stress at a Depth: We have to define only four parameters to fully describe the state of stress at depth namely three principal stress magnitudes, and one stress orientation. (i) Sv, the vertical stress, corresponding to the weight of the overburden; (ii) SHmax, the maximum principal horizontal stress; and (iii) Shmin,the minimum principal horizontal stress and (iv) one stress orientation, usually taken to be the azimuth of the maximum horizontal compression, SHmax. This obviously helps make stress determination in the crust (as well as description of the in situ stress tensor)a much more tractable problem. E. M. Anderson s Classification Scheme: We have to consider the magnitudes of the greatest, intermediate, and least principal stress at depth (S1, S2, and S3) in terms of Sv, SHmax and Shmin in the manner originally proposed by E. M. Anderson. As illustrated in Figure 1.1 and Table 1.1, the Anderson scheme classifies an area as being characterized by normal, strike-slip or reverse faulting depending on whether (i) the crust is extending and steeply dipping normal faults accommodate movement of the hanging wall (the block of rock above the fault) downward with respect to the footwall (the block below the fault), (ii) blocks of crust are sliding horizontally past one another along nearly vertical strike-slip faults or (iii) the crust is in compression and relatively shallow-dipping reverse faults are associated with the hanging wall block moving upward with respect to the footwall block. (iv) The Anderson classification scheme also defines the horizontal principal stress magnitudes with respect to the vertical stress. (v) The vertical stress, Sv, is the maximum principal stress (S1) in normal faulting regimes,the intermediate principal stress (S2) in strike-slip regimes and the least principal stress(s3) in reverse faulting regimes. International Journal of Science Technology & Management Page 57

4 Figure 1.1. E. M. Anderson s classification scheme for relative stress magnitudes in normal, strike-slip and reverse faulting regions. International Journal of Science Technology & Management Page 58

5 According to the Anderson classification scheme, the horizontal principal stresses may be less than, or greater than, the vertical stress, depending on the geological setting. The relative magnitudes of the principal stresses are simply related to the faulting style currently active in a region. As illustrated in above Figure 1.1, the vertical stress dominates in normal faulting regions (S1 = Sv), and fault slip occurs when the least horizontal principal stress (Shmin) reaches a sufficiently low value at any given depth depending on Sv and pore pressure. Conversely, when both horizontal stresses exceed the vertical stress (S3 = Sv) crustal shortening is accommodated through reverse faulting when the maximum horizontal principal stress (SHmax) is sufficiently larger than the vertical stress. Strike-slip faulting represents an intermediate stress state (S2 = Sv), where the maximum horizontal stress is greater than the vertical stress and the minimum horizontal stress is less (SHmax Sv Shmin). In this case, faulting occurs when the difference between SHmax and Shmin is sufficiently large. In-Situ Stress Measurements: The In-Situ Stresses are the three principal in situ stresses namely (S1,S2,S3) and they are determined in the context of (SHmax, Sv, Shmin).The determination of these stress values are very essential in the calculation of induced stresses at the wellbore wall. A general overview of the strategy that will use for characterizing the stress field is as follows: (i) (ii) (iii) Assuming that the overburden is a principal stress (which is usually the case), Sv can be determined from integration of density logs as discussed previously The orientation of the principal stresses is determined from wellbore observations, recent geologic indicators and earthquake focal mechanisms S3 (which corresponds to Shmin, except in reverse faulting International Journal of Science Technology & Management Page 59

6 (iv) (v) regimes) is obtained from minifracs and leak-off tests Pore pressure, Pp, is either measured directly or estimated from geophysical logs or seismic data Having observations of wellbore failures (breakouts and drillinginduced tensile fractures) allows for much more precise estimates of SHmax. Determination of Rock Properties In a borehole stability study it is necessary to define the failure surfaces of the failure criteria used. To do this the elastic properties and strength of the formation examined must be determined. There are two ways of obtaining this information: 1. Laboratory Testing. 2. Borehole Well Log Analysis Laboratory Testing Rock properties are determined by triaxially compressing rock cores. The testing is carried out over a range of axial and confining pressures with some samples being tested to failure. The rock pore pressure and temperature must also be controlled so that the test conditions match those of the rock formation studied as closely as possible. Rock properties measured are: uniaxial compressive strength(co) shear strength (cohesion), C Poisson s ratio(v) tensile strength angle of internal friction (φ) The advantage of laboratory testing is the high level of control maintained over the factors affecting rock stress-strain behaviour and failure, for example temperature, pressure and stress path. However, because of the prohibitive cost of obtaining samples of rock for testing from the in-situ formations of interest, surface outcrop rock of the formation is used. A drawback of this practice is that the mechanical properties of the outcrop formation may not be the same as the in-situ formation. Borehole WellLog Analysis Elastic properties of the rock can be related to the compressive and shear wave velocities through rock, as measured by the sonic log and by the gamma-ray log, The uniaxial compressive strength (Co) is calculated using the above elastic properties from:- where φ = internal friction angle, is taken to be 30, when no experimentally derived value is available, Vclay = clay volume (%) The tensile strength of rock is usually about 1/10th the compressive strength. International Journal of Science Technology & Management Page 60

7 The advantage of using log-derived data to determine rock elastic properties and uniaxial compressive strength is that logs are available for the majority of the borehole length; usually only the top-hole is not logged before running casing. The disadvantage of using log derived data is that the presence of drilling mud has altered the in-situ stress state, and it is known that stress state affects stress-strain behavior and strength. Rock Failure under Compression The borehole fails in compression when the pressure of the drilling mud is insufficient to keep the shear stresses in the borehole wall below the shear strength of the formation. When the borehole fails in compression, broken rock falls into the borehole and the borehole diameter increases at the point of failure. Both the increase in borehole diameter and the volume of rock debris falling into the borehole sometimes make it difficult or impossible to move drilling equipment into or out of the borehole. Certain rock types, such as salt, creep rather than fail when compressed and may close around equipment in the borehole or reduce borehole diameter, again making it difficult or impossible to move drilling equipment into or out of the hole. Rock Failure under Tension The borehole fails in tension when the pressure exerted by the drilling mud induces stresses in the borehole wall that exceed the tensile strength of the rock. The failure takes the form of cracks, typically starting from the borehole wall and running radially into the formation. Drilling mud may then penetrate and propagate these cracks, leading to a fall in mud level in the borehole. If this continues, the borehole stability will eventually be restored by the resulting reduction in the hydrostatic loading of the hole at depth. Wellbore Stability Analysis: Several analytical solutions have been derived for an arbitrary borehole orientation assuming elastic rock behavior. These are generally considered to makeconservative predictions. More sophisticated models based on viscoelastic, elastoplastic and nonlinear approaches have been proposed. These new models are thought to be more realistic than a simple elastic analysis, since rocks rarely behave in a purely elastic manner until ultimate failure. In cases where laboratory testing of cores is possible and well-defined rock properties can be obtained, contemporary rigorous non-linear modelling techniques can be applied. However, in most practical circumstances, the poor definition of key input parameters (i.e., in-situ stresses and rock strengths) justifies at best, a simplistic conservative elastic analysis. In these cases the rock strength is determined by utilising a peakstrength criterion1, (e.g., von Mises, Drucker-Prager, Mohr- Coulomb failure criterion). Once the type and degree of borehole instability has been identified, the next step is to recommend means of improving the situation. Mitigating borehole instability involves changing some of the operational parameters involved, usually mud weight International Journal of Science Technology & Management Page 61

8 and mud composition. The optimum corrective action depends on a proper assessment of the cause of borehole instability and of the effect of the measures contemplated. During the development phase where engineers seek to optimize wellbore stability through determination of optimal well trajectories, casing set points and safe mud weights. The implications of borehole instability to lost drilling time and equipment have prompted operators and service companies to apply Petroleum Geomechanics principles to define working limits for mud weights to avoid tensile or compressive failure. The theoretical analysis involved in borehole stability is quite complex and requires a great deal of mathematical derivation. The quantification of wellbore instability requires the understanding and quantifying of five steps: 1. Determining magnitude and direction of in-situ earth stresses 2. Determining rock properties 3. Establishing a rock failure criterion 4. Calculation of induced stresses around the wellbore for vertical and deviated wells. 5. Compare the induced stresses with the stresses from failure criterion to establish if the wellbore will fail Summary Important factors that contribute to wellbore instability were briefly discussed. Key parameters that influence wellbore instability are pore pressure, rock properties including strength, in-situ stresses, well trajectory and the drilling fluid. The timedependent nature of the stress and pore pressure distributions influenced by drilling fluid exposure, manifests as time-dependent wellbore stability. To better manage wellbore instability problems in the field an in-depth understanding of the fundamental stress concepts more importantly. This technology should be translated into simple practical terms so that field personnel can apply these principles to solve practical problems. References Aadnoy, B. S. and Hansen, A. K., 2005, Bounds on in-situ stress magnitudes improve wellbore stability analyses, SPE Journal, 10 (2), Bell, J. S., 2003, Practical methods for estimating in situ stresses for borehole stability applications in sedimentary basins, Journal of Petroleum Science and Engineering, 38, Belonin, M. D., Smirnova, E. M., Slavin, V. I., Chilingar, G. V. And Robertson, J. O., 2003, Exploration and exploitation in oil and gas fields with abnormally high formation pressures, Energy Sources, 25, International Journal of Science Technology & Management Page 62

9 Chatterjee, R. and Mukhopadhyay, M., 2002, Petrophysical and Geomechanical properties of rocks from the oil fields of the Krishna- Godavari and Cauvery basins, India, Bulletin of Engineering Geology and the Environment, 61, Chatterjee, R. and Mukhopadhyay, M., 2003, Stress modeling for the Oil and Gas fileds of Krishna-Godavari and Cauvery basins, India, using Finite Element Technique, Petrophysics, 44(5), Djurhuus, J. and Aadnoy, S. S., 2003, In situ stress state from inversion of fracturing data from oil wells and borehole image logs, Journal of Petroleum Science and Engineering, 38, Gowd, T. N., Rao, S. V. S. and Gaur, V. K., 1992, Tectonic stress field in the Indian subcontinent, Journal of Geophysical Research, 97 (B8), 11,879-11,888. Tare, U.A., and Mody, F.K.: Novel Approach to Borehole Stability Modeling for ERD and Deepwater Drilling, paper SPE 52188, 1999 SPE Mid-Continent Operations Symposium, Oklahoma City, USA, March 28 31, International Journal of Science Technology & Management Page 63