Clarkson, Christopher R. (University of Calgary) | Wood, James (Encana Corporation) | Burgis, Sinclair (Encana Corporation) | Aquino, Samuel (University of Calgary) | Freeman, Melissa (University of Calgary)
The pore structure of unconventional gas reservoirs, despite having a significant impact on hydrocarbon storage and transport, has historically been difficult to characterize because of a wide poresize distribution (PSD), with a significant pore volume (PV) in the nanopore range. A variety of methods is typically required to characterize the full pore spectrum, with each individual technique limited to a certain pore size range. In this work, we investigate the use of nondestructive, low-pressure adsorption methods, in particular low-pressure N2 adsorption analysis, to infer pore shape and to determine PSDs of a tight gas siltstone reservoir in western Canada. Unlike previous studies, core-plug samples, not crushed samples, are used for isotherm analysis, allowing an undisturbed pore structure (i.e., uncrushed) to be analyzed. Furthermore, the core plugs used for isotherm analysis are subsamples (end pieces) of cores for which mercury-injection capillary pressure (MICP) and permeability measurements were previously performed, allowing a more direct comparison with these techniques. PSDs, determined from two isotherm interpretation methods [Barrett-Joyner-Halenda (BJH) theory and density functional theory (DFT)], are in reasonable agreement with MICP data for the portion of the PSD sampled by both. The pore geometry is interpreted as slot-shaped, as inferred from isotherm hysteresis loop shape, the agreement between adsorption- and MICP-derived dominant pore sizes, scanning-electron-microscope (SEM) imaging, and the character of measured permeability stress dependence. Although correlations between inorganic composition and total organic carbon (TOC) and between dominant pore-throat size and permeability are weak, the sample with the lowest illite clay and TOC content has the largest dominant pore-throat size and highest permeability, as estimated from MICP. The presence of stress relief-induced microfractures, however, appears to affect laboratory-derived (pressure-decay and pulse-decay) estimates of permeability for some samples, even after application of confining pressure. On the basis of the premise of slot-shaped pore geometry, fractured rock models (matchstick and cube) were used to predict absolute permeability, by use of dominant pore-throat size from MICP/adsorption analysis and porosity measured under confining pressure. The predictions are reasonable, although permeability is mostly overpredicted for samples that are unaffected by stressrelease fractures. The conceptual model used to justify the application of these models is slot pores at grain boundaries or between organic matter and framework grains.
Alaskar, Mohammed N. (Stanford University) | Ames, Morgan F. (Stanford University) | Connor, Steve T. (Stanford University) | Liu, Chong (Stanford University) | Cui, Yi (Stanford University) | Li, Kewen (Stanford University) | Horne, Roland N. (Stanford University)
The goal of this research was to develop methods for acquiring reservoir pressure and temperature data near the wellbore and farther out into the formation and to correlate such information to fracture connectivity and geometry. Existing reservoir-characterization tools allow pressure and temperature to be measured only at the wellbore. The development of temperature- and pressure-sensitive nanosensors will enable in-situ measurements within the reservoir. This paper provides the details of the experimental work performed in the process of developing temperature nanosensors. The study investigated the parameters involved in the mobility of nanoparticles through porous and fractured media. These parameters include particle size or size distribution, shape, and surface charge or affinity to rock materials.
The principal findings of this study were that spherically shaped nanoparticles of a certain size and surface charge compatible with that expected in formation rock are most likely to be transported successfully, without being trapped because of physical straining, chemical, or electrostatic effects. We found that tin-bismuth (Sn-Bi) nanoparticles of 200 nm and smaller were transported through Berea sandstone. Larger particles were trapped at the inlet of the core, indicating that there was an optimum particle size range. We also found that the entrapment of silver (Ag) nanowires was primarily because of their shape. This conclusion was supported by the recovery of the spherical Ag nanoparticles with the same surface characteristics through the same porous media used during the Ag nanowires injection. The entrapment of hematite nanorice was attributed to its affinity to the porous matrix caused by surface charge. The hematite coated with surfactant (which modified its surface charge to one compatible with flow media) flowed through the glass beads, emphasizing the importance of particle surface charge.
Preliminary investigation of the flow mechanism of nanoparticles through a naturally fractured greywacke core was conducted by injecting fluorescent silica microspheres. We found that silica microspheres of different sizes (smaller than the fracture opening) could be transported through the fracture. We demonstrated the possibility of using microspheres to estimate fracture aperture by injecting a polydisperse microsphere sample. It was observed that only spheres of 20 µm and smaller were transported. This result agreed reasonably well with the measurement of hydraulic fracture aperture (27 µm), as determined by the cubic law.
Acharya, Mihir Narayan (Kuwait Oil Company) | Kabir, Mir Md Rezaul (Kuwait Oil Company) | Al-Ajmi, Saad Abdulrahman Hassan (Kuwait Oil Company) | Pradhan, San Prasad (Kuwait Oil Company) | Dashti, Qasem M. (Kuwait Oil Company) | Al-anzi, Ealian H.D. (Kuwait Oil Company) | Chakravorty, Sandeep (Schlumberger)
The deep, sub-salt reservoir complex is tiered with fractured tight carbonate at bottom and top, with the two sub-units of "upper unconventional kerogen?? and "lower inter-bedded kerogen-carbonate?? in the middle. This depositional setting is challenging for horizontal well placement where the thicknesses of respective sub-units are about 50 and 30 feet with varying geomechanical and petrophysical properties. Additionally, this complexity poses limitations in completions and effective stimulation of the Kimmeridgian-Oxfordian reservoirs in several gas fields at development stage in Kuwait.
A horizontal well is placed in the lower sub-unit of the laminated complex of unconventional kerogen and fractured carbonate reservoir as a Maximum Reservoir Contact (MRC) type well. A pilot mother-bore was drilled and logged to identify the lithological properties across the entire vertical domain - facilitates the optimization of horizontal drain-hole placement within the targeted reservoir units.
No wellbore stability issues in drilling were predicted based on the geomechanical understanding where core-calibrated logs from offset vertical wells were considered. However, this modeling method did not have the functionality to integrate the impact of drawdown on the laminated formation which became unstable and collapsed during the short open-hole drill-stem test (DST) plugging the tubing prior to the final completions. An alternative "book-shelf?? geomechanical model was considered at pre-drill stage for predicting the wellbore stability. Once the drilling was completed, the time-lapsed multi-arm caliper indicated the validity of the alternative methodology in predicting the unstable stack of laminations in kerogen-rich strata.
The paper discusses an optimization methodology to enhance the understanding of static and dynamic geomechanical stability through the use of BHI data. Objective of the proposed method is to help improve the effectiveness of completions where wellbore stability due to geomechanical complexity in stacked-pay reservoirs is a primary wellbore challenge in deploying the completions and executing a subsequent stimulation and testing campaign.
Waterflooding has been the most popular post-primary production approach for improving oil recovery. In fractured reservoirs with large structural relief, gas injection can produce much of the post-waterflood remaining oil by gravity drainage. Oil recovery by gas-invoked gravity drainage in waterflooded reservoirs is known as the double displacement process (DDP). One major reason, among many, is that the three-phase relative permeability residual oil saturation endpoint is generally smaller than the residual oil saturation endpoint for the water-oil displacement.
Field data indicate that the DDP has been successful in single-porosity sandstone formations. Intuitively, one can expect that DDP should produce similar results in reservoirs with ample intercommoned vertical fractures, which is the objective of this work. With the aid of tests on tight reservoir cores from a major Middle East carbonate reservoir, this study focuses on evaluating the DDP in fractured carbonate reservoirs where the wettability ranges from neutral to oil-wet conditions. The scope of the study includes: (1) assessment of the DDP experimentally in fractured cores using a high-speed centrifuge, (2) simulating the experiments numerically, and (3) upscaling laboratory results to field applications.
Results from water-oil gravity drainage tests followed by gas-oil gravity drainage in fractured and unfractured cores are presented. We also show numerical simulation results of matching the experiments using both transfer function and 2-D numerical simulation, and how results from our study can be used in field applications.
Typical waterflood oil recovery from 0.1-md to 2-md fractured carbonate cores has been noted to be around 38% of the initial oil in place while incremental additional oil recovery for gas-oil gravity drainage is nearly as much as the recovery from water.
Methodologies and numerical tools are available (1) to construct geologically realistic models of fracture networks and (2) to turn these models into simplified conceptual models usable for fieldscale simulations of multiphase production methods. A critical step remains however, that of characterizing the flow properties of the geological fracture network. The multiscale nature of fracture networks and the associated modeling cost impose a scale-dependent characterization: (1) multiscale fractures that may be characterized in local dynamic test areas, e.g., drainage areas involved in well tests, through the calibration of geologically realistic fracture models; and (2) large-scale faults that are characterized through reservoir-scale production history simulations that involve upscaled flow models with an explicit fault representation. However, field data are commonly insufficient to fully characterize the multiscale fracture properties. Therefore, efficient inversion methodologies are necessary to sample wide ranges of property values and to characterize a variety of solutions, i.e., fracture models that are consistent with dynamic data. This article presents an inversion methodology to facilitate the characterization of fracture properties from well-test data. A genetic optimization algorithm has been developed and coupled with a three-dimensional fracture model upscaling simulator to perform the simultaneous calibration of well-test data, i.e. equivalent transmissivities K·h, with K the equivalent permeability that takes into account fracture flow properties, and h the reservoir thickness over which the well test has been interpreted. Several genetic crossover and mutation strategies were studied and tested on three geologically realistic fractured reservoir models, involving both small-scale diffuse fractures and large-scale sub-seismic faults. The characterized diffuse fracture properties are mean length, mean conductivity, orientation dispersion factors, and facies-dependent properties such as fracture density. The fault network conductivity is also characterized. The effectiveness of this inversion methodology to characterize physically meaningful and data-consistent fracture properties is discussed.
Inverse problem, genetic algorithm, KH calibration, fracture, characterization
Geological models are designed from the integration of various type of data (seismic data, well data, outcrop observations...) . These data are usually sparse, scarce or scale-limited, i.e. they do not allow one to fully characterize the geological features in a given area of interest. Therefore geological models also result from many assumptions made on various parameters distributions, with associated space-variable uncertainties. To reduce these uncertainties, flow-related data are also used, geologically-constrained flow models are designed and flow simulations are performed . Uncertain parameters are characterized via calibration of the simulated results with flow-related data. The calibrated flow models are then used for estimating the reservoir production capacity and developing strategies for optimizing the production  In particular, fractured reservoirs are difficult to characterize, mostly due to the multiscaled character of fracture network and their high degree of heterogeneity . Specific workflow and tools have been developed for fractured reservoir characterization in the past years . Although some progress have been made in fracture detection, the presence of fractures at various scales affects the flow dynamics within a reservoir, and casts large uncertainties on the assessment of the field productivity and reserves. This paper focuses on the characterization of fracture properties from well tests. Well tests are perturbations of well rates over a given period of time, during which the well pressure response is recorded. The inverse problem is then to infer the fracture properties from well tests pressure measurements.
Edwards, John Ernest (Schlumberger) | Herrera, Adrian (Schlumberger) | Judd, Tobias Conrad (Schlumberger) | Kristensen, Morten Rode (Schlumberger Middle East SA.) | Al-Rashdi, Yaqoob Salem (Petroleum Development Oman) | Hindriks, Cornelius
A new method of stress testing with a wireline tool uses a drilled hole sealed with a compression pad to apply hydraulic pressure to the reservoir. Geomechanics modeling shows why this reduces the fracture initiation pressure and avoids intersecting the borehole with the induced fracture. The superposition of two induced stresses, mechanical and hydraulic, causes the tensile failure to initiate towards the end of the drilled hole as the hydraulic pressure is increased. The 9-mm-diameter hole is drilled from the center of the sealing compression pad to a depth of up to 15 cm. A fracture initiating some distance from the wellbore will be located part way through the near-wellbore perturbed stress field and will propagate away from the wellbore to the far field in the direction of reduced minimum stress. Reservoir simulation shows that the leakage rate of injected fluid around the compression pad is insignificant.
The first jobs using this technique are described, including procedures for passive tool orientation so that the drilled hole is aligned with the maximum horizontal stress. Information revealed about breakdown pressures in tight dolomite explained why drilling-induced fractures were affecting resistivity logs and well test interpretation. The current procedure for stress testing is the pumping of drilling mud between inflated packers. The new technique described here solves two problems associated with the inflated packer method. The pumped fluid volumes are much smaller, so clean fracture fluid from a sample chamber can be used instead of mud. And the system compressibility is reduced, so the pressure transients are more responsive to the formation.
The ability to induce a fracture in the formation with a pad tool using dedicated fluid with a low dead volume creates a new way of connecting to the reservoir, an alternative to connecting via the borehole wall surface. This large, undamaged contact area due to the induced fracture beyond the drilling-damaged zone will facilitate sampling low-permeability formations or high-viscosity oils.
Our studies of the underlying fundamental gas-recovery mechanisms from shale gas are motivated by expectations of the increasing role of shale gas in national energy portfolios worldwide. We use pore-scale analysis of reservoir shale samples to identify critical parameters to be employed in a gas-flow model used to evaluate well-production data. We exploit a number of 3D-imaging technologies to study the complexity of shale pore structure: from low-resolution X-ray computed tomography (CT) to focused ion beam and scanning electron microscopy (FIB/SEM). We observe that heterogeneity is present at all scales. The CT data show fractures, thin layers, and density heterogeneity. The nanometer-scale-resolution FIB/SEM images show that various mineral inclusions, clays, and organic matter are dispersed within a volume of few-hundred µm3. Samples from different regions differ sharply in the shape, size, and distribution of pores, solid grains, and the presence of organic matter. Although the samples have clearly distinguishable signatures related to the regions of origin, extremely low permeability is a common feature. This and other pore-scale observations suggest a bounded-stimulated-domain model of a horizontal well within fractured shale that accounts for both compression and adsorption gas storage. Using the method of integral relations, we obtain an analytical formula approximating the solution to the pseudopressure diffusion equation. This formula makes fast and simple evaluation of well production possible without resorting to complex computations. It ss a decline curve, which predicts two stages of production. During the early stage, the production rate declines with the reciprocal of the square root of time, whereas later, the rate declines exponentially. The model has been verified by successfully matching monthly production data from a number of shale-gas wells collected over several years of operation. With appropriate scaling, the data from multiple wells collapse on a single type curve. Pore-scale image analysis and the mesoscale model suggest a dimensionless adsorption-storage factor (ASF) to characterize the relative contributions of compression and adsorption gas storage.
This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 157031, "Application of Nanotechnology in Drilling Fluids," by Katherine Price Hoelscher, SPE, Guido De Stefano, SPE, Meghan Riley, SPE, and Steve Young, SPE, M-I SWACO, prepared for the 2012 SPE International Oilfield Nanotechnology Conference and Exhibition, Noordwijk, The Netherlands, 12-14 June. The paper has not been peer reviewed.