EAGE/AAPG Middle East Tight Gas Reservoirs Workshop 2011

3-D geomechanical model and coupled geomechanics modeling between geomechanical and reservoir simulators are required for investigating and quantifying the full 3-D nature of in-situ stresses, pore pressure and rock properties, and their spatial and temporal variations. Applications of such 3-D geomechanical model and coupled geomechanics modelling include challenging development and infill well drillings, reservoir compaction, overburden movement and fault re-activation which could impact long term openhole stability. The presentation will introduce novel mud weight cubes for analysis of multiple well trajectories and selection between alternative trajectories for a planned well. The mud weight cubes can also be used for field scale drilling optimization including identification of regions in the field which pose high drilling risks, planning of well locations, optimization of well trajectories etc. in field scale, taking wellbore stability into consideration. In addition, geomechanics of depletion, reservoir compaction, subsidence, fault re-activation and overburden movement as well as their effects/consequences on long term openhole stability will be described. Case studies utilizing 3-D drilling geomechanics and coupled reservoir geomechanics modeling, and applications of the technologies to drilling and long term openhole stability will also be presented and discussed.

Coupled geomechanics modeling is required for investigating and quantifying the full 3-D nature of the in-situ stresses, pore pressure and rock properties, and their spatial variation. This enables the complex subsurface conditions and properties through the overburden and reservoir to be modeled and evaluated, and then used in subsequent geomechanics analyses, well planning and reservoir management. Pore pressure and stress changes (which may occur in both the overburden and reservoir) due to injection, production and depletion can be quantified and forward modeled, and the coupling of the petrophysics to the geomechanics means that dynamic reservoir behaviour can also be modeled and investigated. Applications of such 3-D modeling and fully coupled analyses include pre-production and infill well planning, wellbore stability, completion design, stimulation, injection, waste and CO2 disposal, reservoir engineering, 4-D seismic, reservoir compaction, subsidence, fault activation and induced seismicity. The presentation will introduce geomechanics and consequences of depletion and injection, reservoir compaction, overburden movement, subsidence, caprock integrity and fault re-activation. Case studies utilizing coupled reservoir geomechanics implementation and applications of this technology to optimal field development planning for life of field will be presented and discussed.

This study is an attempt to shed some light on the well testing interpretation challenges of complex fluvial environments, and suggest interpretation and provide numerical well testing models to the pressure data. Four field cases are presented, a well drilled on the flank of a meandering channel, sand lenses effect on pressure data, characterization of a compartmentalized sand reservoir and a well intersecting a fracture network. In the latter case, two examples of a fracture behavior at early times and a conductive fault behavior at late times were identified. Because of the complex nature of these flow problems, the power of numerical simulation along with integrating all available data, including seismic, approach was used to match the pressure responses.

Tight gas reservoirs have many unique challenges associated with the drilling, completion practices required to obtain economic production rates. Formation damage is an undesirable operational and economic dilemma that may occur during any phase of gas recovery from tight gas reservoirs. Prevention, control and remediation of formation damage are among the most important issues to be resolved for efficient exploitation of tight gas reservoirs. Designing certain chemicals and/or treatment procedures for damage control and remediation is not an easy scientific and engineering task. Recipes that work for certain cases may not necessarily work for others. Formation damage mechanisms affecting tight gas reservoirs, e.g. relative permeability and capillary pressure effects, will be discussed in this paper. A review of commonly practiced methods and tools available for prevention, control and remediation of formation damage will be presented.

Monitoring reservoir stimulation operations provides data for predicting production performance and for reservoir characterization but also, potentially, for compliance with local regulations. With improved drilling and completion technology the depth of the unconventional reservoirs produced increases and the options for deploying cost–effective microseismic monitoring equipment become limited. The monitoring technology has to adapt by optimizing acquisition geometry and data processing as well as the procedures that demonstrate the validity of the results. A practical solution for microseismic monitoring of stimulation operations in an unconventional reservoir under development is a surface or shallow distributed array. We are analyzing two such datasets together with complementary deep borehole sensor datasets in order to understand how to predict and validate the expected performance of distributed surface and shallow arrays. The surface recorded data is processed by stacking and event detection and location are accepted based on statistical criteria. This catalog of events is compared to the one obtained from borehole array where waveforms can be analyzed individually in order to validate the quality of event analysis.

Reservoir sweet spots are areas with better permeability relative to the surrounding country rocks. Theyrepresent a major opportunity in exploration and development of tight gas reservoirs. This paper describes the types of reservoir sweet spots in the Arabian basin and the most common factors controlling their development. Three types of reservoir sweet spots are identified in the Arabian basin; tectonic, sedimentologic and diagenetic sweet spots. The tectonic sweet spots are areas with open fractures being the main control of reservoir productivity. The sedimentologic sweet spots are areas with coarser-grained and possibly thicker sandstone than the surrounding clastics, or areas where porous carbonate build-ups exist. The diagenetic sweet spots are areas with improved permeability due to re-crystallization, dissolution, dolomitization, or early grain coating caused by the flowingfluids. The reservoir sweet spots result from the interplay of one or more of three factors; a) local redistribution of tectonic stresses at the heterogeneous basement fabrics, b) combined effect of sea level changes and sediment influx, and c) diagenetic processes acting on reducing rock volume and increasing its brittleness. Integrated geologic analysis of these factors is a key element to predictpotential locations for sweet spots.

etion. The presentation will focus on two main topics : • An integrated workflow that combines results from 4D seismic inversion, geomechanics modelling and reservoir simulations in a shared earth model • Application to the Elgin and Franklin HP/HT fields located in the UK Central Graben of the North Sea : o Interval time-shifts using an in-house warping method o A coupled faulted reservoir and geomechanical models to determine both the stress redistribution within and around the reservoir including associated reactivated fault slips, and to derive synthetic 4D time-shifts with the help of a rock physics model o Closing the loop by achieving reasonable match between the observed and the synthetic 4D time-shifts. This integrated study proved to be a useful tool for progressing understanding of the reservoir and production behaviour of the Elgin and Franklin Fields, with the ultimate goal being identification of new drilling targets, mitigation of casing integrity risks, safe infill drilling.

The exploration and exploitation of unconventional gas resources is an expanding trend and is an interesting topic especially within the context of the international marketplace. Unlike the North American tight gas business, where thousands of wells are drilled yearly and the operational infrastructure is proven and in place, the international gas market challenges are significantly different. Rather than enjoying tens of wells to confirm reservoir properties and efficient completion techniques, most international tight gas projects have five or less wells to prove concept sustainability. Subsequently, the key risk is not the hydrocarbon resource itself, but instead the early recognition of areas where critical geological and reservoir factors exist to enable economic and sustainable field development plans. Additionally, the rapid identification and application of the most efficient completion solution is paramount to progress of an economically sanctionable project. Hydraulic fracturing is a critical component of the completion process when appraising, developing, and producing tight gas reservoirs. However, for most regions outside of the large and well established stimulation markets, business and operational success for both operators and service providers is dependent on the ability to maximize the number of synergies while managing an integrated service offering. Therefore, the development of a tailored and carefully planned workflow; that enables speedy evaluation of both the geological and completion performance is essential. Tight gas reservoir understanding is not only limited to presence of hydrocarbons, reservoir distribution, fluid saturation, reservoir contacts, and other geological aspects but is also equally dependent on gas inflow performance (via hydraulic fracturing), ensuring economically attractive well test flow rates and confirmation of sustainable long term production. This generally includes a requirement for an extended well-test in order to confirm the long term reservoir/well behaviour. As a result, an appropriate technical and operational evaluation program for tight gas reservoirs was developed for this project that allowed for several wells to potentially demonstrate the extent, sustainability and nature of reservoir variability and productivity. This presentation will illustrate, the workflow process also incorporates evaluation of these treatments in order to quantify hydraulic fracturing performance and reservoir deliverability. The application of a sound technical strategy, innovative well placement program, and a tailored workflow, provides the greatest chance of success in meeting crucial milestones. It is important that the project stays within technical and budgetary constraints while maintaining flexibility and options for modifications, while not jeopardizing future resource development.

Experience s from three fields involving appraisal and development of Tight Gas Reservoirs (TGR) located in completely different regions share common geomechanical features, especially high in situ stress anisotropy that has a strong impact on drilling and stimulation operations. The presentation will start by introducing stress anisotropy impact in the context of TGR. Geomechanical models of stresses and other mechanical properties for the three fields and the methodology used to determine them will then be shown. Finally, an analysis of the impact of geomechanical conditions on drilling and stimulation operations will be investigated. Among the major issues identified is the optimization of the well trajectory, i.e. azimuth and inclination, according to several criteria that are wellbore stability, especially feasibility of under balanced drilling, interception of natural conductive fracture , success of hydraulic fracturing.

The challenges associated with tight gas developments include, the low initial production rates and the rapid production fall-off within the first years of production for any well. Tight gas developments require a multidisciplinary approach in which each element of a field development is addressed in order to develop the gas resources optimally. Each project will need to optimize the technologies and approaches used in order to successfully develop the tight gas resources. Experience from around the world may be relevant to some of these developments, but direct technology transfer from one field to another may not be relevant when the tight gas resources are on different continents with different geology and geological histories. This paper and presentation will address some of the experience gained in the Middle East and relevant North America experience during the appraisal phases of tight gas developments, focusing on how aspects of Geomechanical Engineering applied to stimulation design can be used to optimize initial production rates. In addition, it will address, as projects move from appraisal to the development stages, how Geomechanical engineering considerations may help in maintaining the production (reducing the production decline curve), through phased and structured development drilling campaigns.