Simulation of Depletion-induced Surface Subsidence in a Coal Seam

Coalbed methane (CBM) drew increasingly the attention as an unconventional source of natural gas during the last decades, globally and domestically. In spite of the fact that it is one of the main hazardous concerns in coal mining it is one of the most advantageous sources of natural gas especially due to its high purity of methane and quality. In conventional natural gas reservoirs the pressurized gas is stored in porous space or fracture space but in CBM natural gas molecules mainly is adsorbed to coal matrix. Therefore in contrast to conventional natural gas reservoirs, the gas production of CBM initiates after decreasing the reservoir pressure down to a threshold in order to initiation the desorption process. According to the presence of water in CBMs which creates a remarkable pressure due to hydrostatic head of water the above desorption threshold will be achieved after dewatering process. Dewatering process will lead in decreasing reservoir pressure in one hand which helps the gas desorption but will lead in increasing effective stress which is applied to rock solid skeleton on the other hand. Such an increase in effective stress accounts for rock structure deformation which has a high impact on surface subsidence due to shallow depth of coal seams. Presence of soft formations in dewatered horizon especially coal seams will increase effectively the deformation of the formations, which could potentially result in remarkable subsidence profile.Studying the depletion induced deformation due to CBM production is the main aim of this study. A three-dimensional finite element program developed will be used to investigate the stress field perturbation and rock structure deformation with emphasize on surface subsidence. In order to cover a wide range of real condition in CBM production a sensitivity analysis is carried out on main parameters including coal seam thickness and deformability properties

A rock mechanical model developed for a Coal Seam Well

Drilling operation in order to produce from Coalbed methane (CBM) is prone to various geomechanics related problems not only within the coal seam but also across the overburden layers. Wellbore instability in the form of shear failure (breakout) and washout in one hand and mud loss and fracturing in other hand are examples of failures which a wellbore may experience if a proper mud weight is not used for drilling. In order to conduct such an analysis the input data required includes mechanical properties of formations as well as the magnitude and direction of in-situ stresses and pore pressure. It is well known that mechanical properties of formations are related to their physical characteristics. For example, the formation Young’s Modulus or strength is expected to be higher in formations with larger sonic velocities or lesser porosities. Petrophysical logs reflect various rock physical properties from which continuous curves of rock mechanical properties could be estimated using several correlations developed in similar fields. Similarly, continuous logs of in-situ stresses (i.e. vertical as well as minimum and maximum horizontal stresses) could be estimated, for example from poroelastic formulae, in conjunction with rock physical properties. The estimated logs could be calibrated against lab tests on cores and field test data. For example, performing triaxial tests in the lab on cores obtained at different depths, the elastic and strength properties such as Young’s Modulus, Poisson’s ratio and uniaxial compressive strength (UCS) could be measured and this is used to correct the corresponding estimated logs. Similarly, the minimum horizontal stress log could be calibrated against any existing leak-off-test data whereas pore pressure curve can be calibrated if any MDT data is available.The direction of horizontal stress can be estimated from the image logs, for example FMI. The combination of continuous curves of formation mechanical properties and magnitude of in-situ stresses together with stress directions is referred to as rock mechanical model (RMM). The RMM is constructed for a drilled well and then it is used for prediction of events in a new planned well in a nearby area. The RMM includes the input data for any geomechanics study such as wellbore instability analysis, fracturing design or sanding prediction. In this study the RMM was constructed for data corresponding to Well Ridgwood 2 drilled in Surat basin in Queensland, Australia. The results indicate how the mechanical properties are changing across the coal seam comparing to other intervals and that the stress magnitudes experience significant changes accordingly. The results are used to predict the fraccability of the CBM for stimulation purposes using a hydraulic fracturing operation. Other applications of the constructed RMM will be discussed and the results interpreted

Design of an ultra-speed Lab-Scale drilling rig for simulation of high speed drilling operations in hard rocks

Drilling is a common process in mining and petroleum engineering applications which have different objectives. For example, drilling deep boreholes in tight gas formations and gas shales is becoming more popular in the oil and gas industry. By contrast, drilling deep small sized holes for mineral exploration in hard rocks is performed to obtain samples for grade analysis purposes. In both applications optimising the drilling process includes using the most effective operating parameters such as rotation speed and weight-on-bit in order to maximise the rate of penetration. Obtaining the optimum drilling parameters requires sensitivity analysis over a range of data, which would be costly and time consuming during real field operations. Therefore, conducting several experimental simulations in the lab would be very beneficial before field operations begin.A drilling rig was designed and developed to simulate various drilling scenarios. The rig works in conjunction with an existing true triaxial stress cell (TTSC) which is in use for different petroleum related applications. The rig allows drilling into a cubic rock sample of up to 300 mm size. Three independent stresses can be applied to the sample to simulate real in-situ field stress conditions while the sample may be saturated with fluid. A significant feature of the rig is its ultra-high speed rotation which can rotate up to 8000 rpm: this is to simulate hard rock drilling for mineral exploration applications where large weight-on-bit could damage the bit cutters. With the proposed design, a drilling fluid of any type can be circulated in the simulated borehole similar to the real situation, to study its effect on drilling performance. The TTSC drilling lid is equipped with torque and drag measurement systems which are two important drilling parameters to be recorded during drilling operation.It is believed that this new drilling rig will open opportunities for performing major new applied research in the area of more efficient drilling in both oil and gas bearing formations and mineral exploration applications

Applications of Underbalanced Fishbone Drilling for Improved Recovery and Reduced Carbon Footprint in Unconventional Plays

Fishbone Drilling (FbD) consists of drilling several micro-holes in different directions from the main vertical or deviated wellbore. Similar to multilateral micro-hole drilling, FbD may be used to enhance hydrocarbon production in naturally fractured formations or in refracturing operations by interconnecting the existing natural fractures. When combined with underbalanced drilling using a coiled tubing rig, FbD enhances the production further by easing the natural flow of the hydrocarbon from the reservoir to the wellbore. The design aspects of the Fishbones include determining the number, length, distance between the branches, and the angle of sidetracking of the branches from the main borehole. In addition, the design of efficient drill string components to suit the FbD conditions are another important design aspect in FbD technology development. Examples of this include a high-performance small, diameter downhole motor and the use of High Voltage Pulsed Discharge (HVPD) plasma shock waves at different pulse frequencies and wave pressures to impose shear forces on the formation to break it more easily. This paper will present a comprehensive review of the FbD technology, including some of its current applications and design aspects. The possibility of using FbD in conjunction with hydraulic fracturing to boost production by creating a network of connected fractures will be discussed, and some of its technical and economic benefits and challenges will be compared

Proposing a sample preparation procedure for sanding experiments

The Authors, during past few years, have performed research on sand production under true triaxial stress conditions. To simulate sanding, 100x100x100 mm3 cubic samples were placed in a true triaxial stress cell (TTSC) and three independent stresses were applied while the pore pressure was increased inside the cell. This resulted in sand grains to be produced through a drilled hole in the sample centre. The experiences obtained through testing several synthetic samples have indicated the significance of sample preparation to obtain valid results. Therefore, in this paper the procedure for preparation of synthetic samples suitable for a sanding experiment is proposed. Also, details of sample preparation for conventional rock mechanical tests to estimate rock physico-mechanical properties including deformability properties, strength parameters and permeability will be presented

Coiled Tube Turbodrilling: A proposed technology to optimise drilling deep hard rocks for mineral exploration

The need to drill deep boreholes more efficiently for mineral exploration has raised the attention to investigate the feasibility of recent drilling technologies for such applications. The two principal methods of Reverse Circulation (RC) and diamond core drilling are usually used in combination by mine operators are subjected to certain limitations and inefficiencies. Considering that delivering large volume of reliable samples from deep zones to the surface in shortest possible time is of paramount importance in mineral exploration, then drilling small size holes as fast as possible and delivering the small chip samples to the surface would be a good alternative with several advantages over conventional drilling methods. As a result, the Coiled Tube (CT) turbodrilling technology is proposed here followed by presenting detailed calculations for the system required power and hydraulics and also Bottom Hole Assembly (BHA) selection suitable for hardrocks mineral exploration applications

True Triaxial Strength Testing of Sandstones

Laboratory rock mechanical tests allow estimation of rock strength and deformation behaviour under stress states similar to the in-situ conditions. In general, the in-situ stresses are described by three principal stresses, the vertical, maximum and minimum horizontal stresses. However, most of rock mechanical properties are obtained using only two different stresses, as in conventional triaxial tests where an axial load and an isotropic confining pressure are applied on a cylindrical rock sample. Also the most commonly used failure criterion, the Mohr-Coulomb criterion, is usually applied using only the maximum and minimum applied stresses and thus ignores the effect of the intermediate stress. Experimental and theoretical studies of rocks under true triaxial stress conditions have proved that describing their mechanical properties while ignoring the effect of σ, cannot reflect the rock behaviour under true stress states. In this paper the lab results of an on-going study on deformation behaviour of synthetic sandstones in a true triaxial cell are presented. The effect of both σ and σ has been examined by conducting compressional tests in different stress levels and σ /σ ratios. The results show the impact of changing stress magnitudes and anisotropy on rock strength and deformation behaviour

Significance of compressional tectonic on pore pressure distribution in Perth Basin

The Perth Basin is one of the major tectonic structures along the western continental margin of Australia and was initially formed through the rifting and break-up of the Indian and Australian plates. The severe tectonic movements accompanied and occurred after the break-up are responsible for the most structural elements and for the distribution of pore pressure in the basin. Investigations on the well log data from the Perth Basin have identified shale intervals which are characterised as overpressured in some parts of the basin, whereas similar shale intervals found to be normally pressured in other parts of the basin. The phenomena of overpressure have frequently been reported while drilling the same intervals. Based on this research, sections with overpressure were observed in the majority of the wells in the basal section of the Kockatea shale where there were less tectonic activities have been recorded. Normal pore pressure was observed in shallower wells in the Kockatea shales which were located within uplifted sections that were more tectonically active areas. Based on the results of this research, the pore pressure distribution in the Kockatea Shale varied significantly from one part of the Perth Basin to another as a result of compressive tectonic stress. Compressional tectonic activities either induced fracturing in shallower localities (e.g. Beagle Ridge, Cadda Terrace and the adjacent terraces) or removed part of the Kockatea Shale as a result of faulting resulting in overpressures being released. Regions with less intensity of the tectonic activities showed an increase in pressure gradients as approaching away from the centre of uplift

Design of a slurry loop for cuttings transport studies in hard rock drilling applications

Transportation of the fluid and slurry (fluid and solid particles mixture) in the pipe and annulus space has been the focus of numerous studies. There are different parameters to be considered when studying slurry transportation. These include slurry velocity or flow rates; fluid properties such as density and rheology; and solid particles properties including concentration, density, shape and size. Also the angle of the flow conduit, rotation of the pipe and possible eccentricity of the annulus are other factors which influence slurry transport characteristics. Although a number of analytical, numerical and empirical equations as well as numerical simulations have been developed for studying the flow and slurry transport, the results need to be validated against either field or lab data. As performing field tests is costly and time consuming conducting simulations at laboratory scale appears as a good alternative. Different flow loops have been designed to study the slurry transport in different science and engineering disciplines including oil and gas and mining. However, few of these consider in particular cuttings transportation in small size annulus space. The flow characteristics appear to be very different when it travels within a small size annulus, in particular when the fluid velocity is high. In this study, a review of some of the existing slurry flow loops will be conducted. Then the details of a slurry loop which has been designed and commissioned for the purpose of studying cuttings transport in a small size annulus space for applications in drilling mineral exploration wells using coil tube technology will be presented

Steps for conducting a valid hydraulic fracturing laboratory test

Several parameters are involved in a hydraulic-fracturing operation, which is a technique used mainly in tight formations to enhance productivity. Formation properties, state of stresses in the field, injecting fluid characteristics, and pumping rate are among several parameters that can influence the process. Numerical analysis is conventionally run to simulate the hydraulic-fracturing process. Before operating the expensive fracturing job in the field, however, it would be useful to understand the effect of various parameters by conducting physical experiments in the lab. Laboratory experiments are also valuable for validating the numerical simulations. Applying the scaling laws, which are to correspond to the field operation with the test performed in the lab, are necessary to draw valid conclusions from the experiments. Dimensionless parameters are introduced through the scaling laws that are used to scale-down different parameters including the hole size, pump rate and fluid viscosity to that of the lab scale. Sample preparation and following a consistent and correct test procedure in the lab, however, are two other important factors that play a substantial role in obtaining valid results. The focus of this peer-reviewed paper is to address the latter aspect; however, a review of different scaling laws proposed and used will be given. The results presented in this study are the lab tests conducted using a true triaxial stress cell (TTSC), which allows simulation of hydraulic-fracturing under true field stress conditions where three independent stresses are applied to a cubic rock sample