Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Deterministic fault re-activation analyses can be a valuable tool for making predictions of induced seismicity risk associated with hydraulic fracturing or waste water disposal. This paper reviews many of the common modelling approaches with this class of geotechnical tools and highlights some of the limitations of the simple analytical models that do not capture all the relevant physics, e.g., poroand thermo-elastic effects, “stress shadows” created during hydraulic fracture treatments, and realistic fault friction coefficients. Using recent data from a well-characterized field in the Montney Formation, NEBC, the sensitivity of fault reactivation during high pressure injection is investigated for the range of measured minimum horizontal insitu stresses and pore pressures from 32 DFIT or mini-frac tests. The importance of site-specific geomechanical data for making confident predictions or post-failure analyses is demonstrated.

A moist coal adiabatic oven test has been used to quantify the effect of applying an anti-oxidant agent to reactive coals from Australia and the US. For the dosage rate applied, the anti-oxidant significantly reduces the coal self-heating rate and extends the time taken to reach thermal runaway by a factor of three for sub-bituminous coal and by a factor of two for the same application to high volatile C bituminous coal. The laboratory result obtained for sub-bituminous coal from Powder River Basin is in direct agreement with the practical site experience of applying the anti-oxidant product as a spontaneous combustion management control. Consequently, it is now possible to benchmark the application of the anti-oxidant to any reactive coal prior to mining as part of developing a leading practice spontaneous combustion management plan.

A moist coal adiabatic oven test has been used to quantify the effect of applying a known inhibiting agent to alter the self-heating rate of highly reactive coals. The inhibitor significantly reduces the coal self-heating rate and extends the time taken to reach thermal runaway by a factor of approximately 3, for the dosage rate applied to the coals. A distilled water placebo test on the most reactive coal using the same application amount increased the initial self-heating rate, but the time taken to reach thermal runaway remained unchanged. These results are in agreement with practical experience and have major implications for the mining, handling and transportation of reactive coals.

A key to the success of long-term storage of CO2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO2 injection zone. Potential leakage mechanisms reviewed include fault reactivation, induced shear failure of the caprock, out-of-zone hydraulic fracturing, and poorly sealed casing cements in enlarged, unstable boreholes. Parameters controlling these mechanisms include the upper and lower bounds of pressure and temperature experienced by the reservoir, the orientation and mechanical properties of existing faults, rock mechanical properties, in situ stresses, and reservoir depth and shape. Approaches to mitigate the likelihood of geomechanics-related leakage include the identification of safe upper limits on injection pressures, preferred injection well locations, review of historical records for reservoir pressures, temperatures and stimulation treatments, drilling program design to mitigate rock yielding in new wells, and assessment of wellbore integrity indicators in existing wells.

D-42 ASSESSING THE FEASIBILITY OF RESERVOIR MONITORING USING INDUCED SEISMICITY Abstract 1 Induced seismicity is a potential monitoring technique to image deformation and fracturing in a number of reservoir applications. This paper describes an integrated geomechanical and geophysical investigation of the feasibility of imaging gas injection and production in a particular reservoir. The results are also generalized for application in other reservoir applications. Introduction In recent years induced seismicity has been used to monitor a number of reservoir operations including but not limited to hydraulic fracturing fault mapping water flooding casing failure waste disposal reservoir compaction and thermal recovery (e.g.

Underbalanced drilling techniques have been applied to avoid or mitigate formation damage, reduce lost circulation risks, and increase the rate of penetration. However, drilling with a bottomhole pressure less than the formation pore pressure will usually increase the risk of borehole instability due to shear or tensile failure of the rock adjacent to the borehole. The extent of rock failure is very sensitive to the pressure in the annulus between the drill pipe, collars or BHA and the formation. The capacity of the drilling fluids to effectively circulate cuttings and cavings to surface is also strongly sensitive to the annular flow velocity. This paper describes the coupling of two popular software packages STABView™ and WELLFLO7™ to solve the complex interaction of borehole instability, rock yielding, collapse, detachment, and wellbore hydraulics during underbalanced drilling operations. In particular, a profile of the average borehole diameter can be predicted that accounts for hole enlargement in weak rock formations, and its consequences for annular pressures and flow velocities.The use of these two models running in a coupled mode is illustrated with two examples. The first one is a case study of a sidetracked well that was drilled underbalanced using coiled tubing technology in western Canada. Severe tight hole problems and poor hole cleaning had been experienced during drilling operations, and ultimately the bottomhole assembly became stuck. Subsequent borehole stability analyses indicated that significant hole enlargement was occurring in two weak shaley intervals. Wellbore hydraulics analyses showed that the liquid velocities achieved in the enlarged intervals of this well were low, which led to an accumulation of cuttings and cavings, thus resulting in the stuck pipe. A second, hypothetical horizontal well case is also described in order to illustrate the procedures for characterizing the operating envelope (i.e., optimal annular pressures and flow rates) prior to drilling an underbalanced well. This example demonstrates additional features of the two software programs for simulating the complexities of hole failure, erosion and enlargement in an annulus with a two-phase fluid for both strong and weak rock cases.

Abstract Sand production and the sand control technique selected to mitigate or eliminate it can have a critical influence on the performance of horizontal wells in gas storage reservoirs. A completion that provides adequate sand control is usually required, but an overly conservative completion design can have an unnecessary, negative consequence on gas well productivity and injectivity. The selection of the appropriate sand control design depends on the characteristics of the reservoir formation, the in-situ stress state, the maximum and minimum values of the reservoir pressure during gas storage operations, the drawdown pressure, the near-well fluid saturations, the well trajectory and the capacity for handling sand in the well tubulars and surface facilities. This paper reviews the principal causes of sand production from borehole and perforation collapse, and demonstrates the application of commercial software programs for assessing sand production risks. The influence of formation damage, compressible and non-Darcy gas flow effects, reservoir pressure changes and rock strength reduction due to cyclic loading will be illustrated. Additional risks, such as exceeding the fracture breakdown pressure in the reservoir during injection, or collapsing weak, interbedded shale strata that are locally penetrated by the horizontal well, are also described. Numerical geomechanical modelling techniques suitable for more complex material behaviour and fluid flow phenomena are also described, as well as a novel procedure for estimating initial produced sand volumes. Several field examples are presented, illustrating results for reservoirs ranging from relatively strong rocks, in which no sand control is required, to poorly cemented sandstones which require gravel-packed or screened completions.

ABSTRACT Borehole instability problems such as stuck pipe, hole enlargement causing poor hole cleaning, and deviation control often arise in the build sections of horizontal wells drilled from surface or as re-entries from existing vertical wells. A drilling fluid system with optimized density, fluid loss, and clay inhibition properties can usually be selected to eliminate or reduce the risk of costly lost time. The selection of such a fluid system depends upon the characteristics of the shales, the in-situ stress state, the planned well trajectory and other well design criteria. This paper reviews the principal causes of mechanical and chemical instability in shales located in build sections and demonstrates several practical software tools and techniques for designing such wells. One international and two Western Canadian field examples in different types of shale will be presented.

Underbalanced drilling techniques are often considered to avoid or mitigate formation damage, reduce lost circulation risk, and increase drilling rate of penetration. However, drilling with a bottomhole pressure less than the formation pore pressure will usually increase the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. Numerous theoretical models for assessing borehole collapse and fracture breakdown risks exist. However, until recently, it has been difficult for non-specialists to use many of these models because I they are not easily implemented, or because they required input parameters that are unfamiliar or difficult to obtain. A user-friendly PC Windows (TM) -based software package called STABView (TM) has been developed to help the well designer determine the optimal range of bottomhole pressure for underbalanced drilling, i.e., the bottomhole pressures that are high enough to avoid severe hole collapse, yet low enough to avoid initiating hydraulic fractures. The software has been designed to perform. rapid parametric analyses for ah types of wells in most geological settings. Guidance in the selection of rock properties and in situ stresses is provided to the user with an online database of typical values and a comprehensive help utility. Applications of the software to underbalanced drilling of horizontal wells in a number of sandstone reservoirs are demonstrated.

The effects of pore pressure penetration and time-dependent rock strength on the growth of the yielded zone around a bore-hole are analysed using an elastic-brittle-plastic model. Solutions for two possible cases are developed: (1) no change in permeability, and (2) a significant increase in permeability upon yielding. The extent of the yielded zone is sensitive to a number of mechanical parameters, of which the residual strength of the rock is most critical. In both cases, the rate of yielded zone growth depends strongly on formation permeability (both prior to and after yielding) and mud filtrate viscosity. In case (2), the initial extent of the yielded zone is also critical to the rate at which it will grow. Model predictions are compared to field data from a western Canadian setting where time-dependent borehole enlargement occurred in a shale interval.

Abstract The mechanical stability of a borehole or perforation tunnels can have a critical influence on petroleum drilling and recovery processes. For instance, extensive failure of rock surrounding an open-hole completion may result in massive sand production. Furthermore, it is often necessary to evaluate the risk of borehole instability when drilling through weak shale cap-rocks, or when drilling underbalanced through poorly cemented reservoir rocks. Numerous geomechanical modeling techniques have been developed to address these issues. However, the existing models are limited in that they fail to account for the inherent variability of mechanical and petrophysical properties of rock, as well as uncertain or poorly constrained values for these and other critical input parameters. This paper describes the adaptation of probabilistic techniques for quantitatively evaluating the risks of borehole instability or sand production. This technique involves the use of a versatile poro-elastoplastic model, implemented within the framework of a spreadsheet-based Monte Carlo simulation tool. The typical results generated by this model indicate that rock strength parameters, bottomhole pressure, reservoir pressure and near-borehole permeability changes have the most significant influence on the extent of yielding for underbalanced drilling or production conditions in weak sandstones. P. 143

This paper investigates the time-dependent effect of drilling fluids on the shear strength properties of Fernie Formation shales. Direct shear tests were conducted on samples that were split along their bedding planes and soaked in four different drilling fluids systems for varying periods of time: oil based mud, gel chem. TAME and PHPA. Distilled water was used as a base line fluid in this study. The shear tests were conducted at a normal stress range of 0-1 MPa. For each fluid, five samples were tested, four of which had their natural rough surface, whereas, the surface of one was artificially smoothened.The test results show that both TAME and PHPA fluids increase the angle of shearing resistance of the smoothened shale samples indicating that these may be absorbed into the shale, at least at the surface. The distilled water, oil based and gel chem muds have no effect on the angle of shearing resistance of the rough samples decreased. The angle of shearing resistance of the rough samples in the TAME fluid showed both increases and decreases, whereas rough samples in the PHPA fluid had a general increase in the angle of shearing resistance with period of soaking.

Wellbore instability can lead to expensive operational problems during the drilling, completion and production of horizontal and inclined wells. This paper reviews the direct and indirect symptoms of wellbore instability, its root causes, and various empirical and deterministic modelling approaches to predicting the risk of hole collapse or convergence. In general, linear elastic models that are only concerned with stability at the wellbore wall often give overly pessimistic predictions. An alternative approach, using the extent of the "yielded" zone around an unstable wellbore and the kinematics of rock detachment, is proposed for practical risk assessments. A case history for an open hole completed horizontal well in a limestone reservoir under high drawdown is described. General guidelines for conducting field-oriented stability assessments conclude the paper.