Bessa, Fadila (Occidental Petroleum Corporation) | Sahni, Vinay (Occidental Petroleum Corporation) | Liu, Shunhua (Occidental Petroleum Corporation) | Tan, Jiasen (Occidental Petroleum Corporation) | Frass, Manfred (Occidental Petroleum Corporation) | Kessler, James (Occidental Petroleum Corporation)
Understanding and modeling the interaction between hydraulic fractures and natural fractures is important to predict shale production performance. This paper presents a workflow that incorporates natural fractures, rock properties, and stress regimes to understand fracture behavior during stimulation treatment. The methodology also integrates the predefined discrete fracture network (DFN) and 3D reservoir properties to build a comprehensive hydraulic fracturing model. Heat maps are also generated to help evaluate completion design and well spacing strategies.
Applying the integrated fracture characterization workflow to the study area revealed that the vertical and lateral fracture growth is a function of structural context, stress conditions, and rock mechanical properties. Stimulation parameters, including proppant volume and injection pressures, for one horizontal and six vertical wells were utilized to build a comprehensive fracture network for the study area. The resulting model shows: (a) the stimulation of predefined natural fractures, and (b) the generation of induced fractures in the maximum stress direction associated with re-activation of pre-existing faults and fractures. The modeling results were validated by interwell interference data.
Fractures play an important role in hydrocarbon production from organic-rich shale reservoirs (Gale, et al., 2014). This is evident from the higher than expected production rates typically observed from low-porosity and ultra-low permeability shale rocks. Moreover, many shale outcrops, cores, and image logs show an abundance of natural fractures or fracture traces. This study integrates natural fracture characteristics, directional stresses, and hydraulic fractures to characterize and better comprehend Permian Wolfcamp production performance.
Several factors influence the stimulated rock volume (SRV) geometry during a hydraulic fracturing stimulation treatment. These factors include: structural context, natural fracture networks, rock mechanical properties, lithology, and stress changes associated with tectonic events (Gale et al., 2014; Maity, 2018). Furthermore, natural fracture systems in shales are heterogeneous; they can enhance or reduce formation productivity, augment or diminish rock strength, and may have a tendency to influence hydraulic fracture stimulation (Doe et al., 2013). The flow of stimulation fluid through natural fractures and the generation of hydraulic fractures were modeled in this study.
Shear reactivation of pre-existing fractures can play a crucial role in hydraulic stimulation to enable production from unconventional reservoirs. Evidence of the phenomenon is found in microseismic/seismic events induced during stimulation by hydraulic fracturing. However, induced seismicity and permeability evolution in response to fracture shear reactivation by injection has not been extensively studied in laboratory tests under relevant conditions. In this work, a cylindrical Eagle Ford shale sample having a single fracture (tensile fracture) was used to perform laboratory injection test with concurrent acoustic emission (AE) monitoring. In the test, slip was induced on the fracture at near critical stress state by injecting pressurized brine water (7% KCL). Sample deformation (stress, displacement), fluid flow (injection pressure, flow rate) and AE signals (hits, events) were all recorded. The data was then used to characterize the fractures’ fully coupled seismo-hydro-mechanical response during shearing. Results shown that the induced AE/microseismic events correlate well with the fracture slip and the permeability evolution. Most of the recorded AE hits and events were detected during the seismic slip interval corresponding a rapid fracture slip and a quick stress relaxation. As a result of dilatant shear slip, a remarkable enhancement of fracture permeability was achieved. Prior to this seismic interval, an aseismic slip interval was evident during the tests, where the fracture slip, the associated stress relaxation, and the permeability increase were limited. The test results and analyses clearly demonstrate the role of shear slip in permeability enhancement by hydraulic stimulation for unconventional shale reservoirs. Finally, it is further revealed that the transition from aseismic slip to seismic slip is highly dependent on the fracture slip velocity.
Shear slip of pre-existing fractures has been recognized as a major permeability creation and micro-seismicity mechanism of reservoir stimulation for a long time (e.g. Pine and Batchelor 1984, Mayerhofer et al. 1997, Rutledge et al. 2004, Zoback et al. 2012). Most reservoir rocks contain abundant pre-existing fractures, some of which may be sealed with calcite or other infill minerals. Usually, these fractures are inactive and without sufficient permeability before stimulation. A number of modeling and field studies have shown that when the pre-existing natural fractures are favorably oriented with respect to the in-situ stresses, an injection or the leak-off from a hydraulic fracture (even with low fluid pressure) can cause the fractures sliding and propping due to asperities. Also, it is widely observed that the induced microseismic or seismic events during hydraulic injection are mostly related to the shear failure of pre-existing fractures. However, fracture slip-permeability evolution and the associated microseismic signature during injection are still poorly understood. The fundamental investigations through laboratory experiments and field injection tests are insufficient.
Rathnaweera, Tharaka D. (Monash University /Nanyang Technological University) | Gamage, Ranjith P. (Monash University) | Wei, Wu (Nanyang Technological University) | Perera, Samintha A. (The University of Melbourne) | Haque, Asadul (Monash University) | Wanniarachchi, Ayal M. (Monash University) | Bandara, Adheesha K. (Monash University)
Over the last several decades, many studies have generated a large amount of proppant performance data, but these studies have only focused on proppant conductivity, with no attention to how proppant mechanical properties vary under loading conditions. The impact of mechanical behaviour on proppant performance can only be fully understood by the combined investigation of micro-structural and mechanical changes with increasing loading. Therefore, this study aims to identify such micro-structural behaviour, and in particular the impact on proppant mechanical properties. Proppant samples were tested under one-dimensional compression loading using high-resolution X-ray CT scanning technology. The reconstructed images taken at different load stages were analysed to capture the micro-structural behaviour and finally correlated with the mechanical behaviour of the proppant.
According to the results, there are significant micro-pore voids inside the proppant mass. When the proppant has a higher degree of porosity, there is a considerable reduction of the compressive strength which is not favourable for hydro-fracturing treatment designs. Moreover, it is clear that the brittleness of the proppant decreases with increasing porosity, as its Young’s modulus reduces with increasing pore voids. Therefore, it is important to have high manufacturing standards to achieve effective proppant performance at great depths. The micro-structural behaviour under increasing loading was investigated by performing comprehensive CT image analysis using Drishti software. According to the results, under compressive loading, proppants cleave and generate large fragments like a flower, and this happens suddenly and quite violently through the material. Interestingly, post-failure analysis revealed that the failure mechanism of a single proppant consists of three major stress levels, where initially proppant fails at a high stress level and gains some crushing-associated strength at later stages.
Unconventional oil/gas production has recently attracted the research community due to the uncontrollable increasing demand for primary energy sources (Perera et al., 2016; Wu et al., 2017). Since this method provides a good solution to energy scarcity, over the last several decades, the industry has tried to enhance the production rate, mainly focusing on production enhancement techniques which can be effectively used in the energy extraction from sub-surface geological formations. Of the various options, hydraulic fracturing is one of the best ways to enhance oil/gas extraction, as it increases the formation’s permeability, allowing easy movement of the extracted oil/gas towards the production well (Rutledge and Scott, 2003; Orangi et al., 2011; Vengosh et al., 2014; Wanniarachchi et al., 2015). However, this process may be jeopardised due to the high stress levels acting on the formation at great depths (both vertical overburden and confining pressures). One possible consequence is re-closure of the fracture network under downhole stress conditions, which severely affects the post-fracturing production. Such issues can negate the use of proppant as a hydraulic fracture treatment method where proppants injected with the fracturing fluid prop the fractures, withstanding the fracture-closure stress (Wanniarachchi et al., 2015). Although the proppant gives a reliable solution to overcome this issue (propping the fracture network), sufficient closure stress can cause mechanical failure of the proppant, changing the fracture conductivity, causing re-closure of the fracture network, and altering the bulk properties of the proppant pack, which can negatively influence oil/gas extraction. Therefore, it is important to understand the mechanical behaviour of proppants under downhole stress conditions before injecting proppant with the hydro-fracturing fluid.
A detailed microseismic survey was carried out in the Spraberry Field of Midland County, Texas. A vertical pilot well was completed in eight separate stages and was monitored from four surrounding wells within 300 m of the completion well. Each monitor well was equipped with a 20-level 3C geophone array spanning the completion zones. A velocity model was built from dipole sonic and calibrated for vertical transverse isotropy (VTI) parameters using perforation shots, sonic-derived anisotropy, and crosswell seismic time picks. The microseismic locations reveal simple vertical planar clouds only a few meters wide and oriented along maximum horizontal stress direction. Moment tensor solutions show a mix of strike-slip and dipslip shear mechanisms along failure planes aligned with the hydraulic fracture orientation. Formation microimager data indicate that natural fractures play little or no role in generating the microseismicity or guiding fracture growth. The observations indicate that the microseismic shearing is directly associated with the hydraulic fractures.
Presentation Date: Wednesday, October 17, 2018
Start Time: 1:50:00 PM
Location: 208A (Anaheim Convention Center)
Presentation Type: Oral
Currently, there are several (four main) widely discussed theories used to describe how microseismicity interacts with hydraulic fracturing. Each theory has a different implication for the interpretation of microseismicity used for reservoir modeling. Therefore, better understanding of the relationship between microseismicity and hydraulic fracture stimulation is needed before further reservoir models are developed and applied. This would lead to a more precise estimation of stimulated rock volume, hydrocarbon production and give greater value to microseismic data. We may use either seismic or non-seismic methods. While non-seismic methods provide an independent view of hydraulic fracturing they only provide a limited amount of information on the relationship between hydraulic fracturing and microseismicity. We propose microseismic monitoring of directivity as the most promising way to determine the orientation of fault planes and associated slip vectors. Although this is a suitable method it requires sensors in multiple azimuths that are well coupled to obtain reliable high frequency signals. We suggest using Distributed Acoustic Sensing (DAS) sensors which are capable of sampling at high frequency and may provide continuous data along long offsets at reasonable costs.
Presentation Date: Tuesday, October 16, 2018
Start Time: 1:50:00 PM
Location: 208A (Anaheim Convention Center)
Presentation Type: Oral
We investigate a novel application of Frechet derivatives for time-lapse mapping of deep, electrically–enhanced fracture systems with a borehole to surface DC resistivity array. The simulations are evaluated for a cased horizontal wellbore embedded in a homogeneous halfspace, where measurements are evaluated near, mid–range, and far from the well head. We show that, in all cases, measurements are sensitive to perturbations centered on the borehole axis and that the sensitivity volume decreases as a function of increased measurement offset from the well head. The sensitivity analysis also illustrates that careful consideration must be taken when developing an electrical survey design for these scenarios. Specifically, we show that positive perturbations in earth conductivity near the wellbore can manifest as both positive and negative measurement perturbations, depending on where the measurement is taken. Furthermore, we show that the transition between the regions along the wellbore of positive and negative contribution results in a "pinch point", representing a region along the wellbore where a given surface measurement is blind to any changes or enhancement of electrical conductivity.
Presentation Date: Wednesday, October 19, 2016
Start Time: 2:20:00 PM
Presentation Type: ORAL
Wilson, Thomas (West Virginia University) | Carr, Timothy (West Virginia University) | Carney, B. J. (Northeast Natural Energy, LLC) | Hewitt, Jay (Northeast Natural Energy, LLC) | Costello, Ian (Northeast Natural Energy, LLC) | Jordon, Emily (Northeast Natural Energy, LLC) | MacPhail, Keith (Schlumberger) | Uschner, Natalie (Schlumberger) | Thomas, Miranda (Schlumberger) | Akin, Si (Schlumberger) | Magbagbeola, Oluwaseun (Schlumberger) | Morales, Adrian (Schlumberger) | Johansen, Asbjoern (Schlumberger) | Hogarth, Leah (Schlumberger) | Naseem, Kashif (Schlumberger)
In this study, we take a preliminary look at microseismic data collected along the length of two Marcellus shale horizontal wells (the 3H and 5H) drilled by Northeast Natural Energy LLC (NNE) in Morgantown, West Virginia. Detailed log data are also available along the length of one of the laterals (the 3H) that provide a wealth of information concerning geomechanical properties, fracture trend and intensity. Logging, processing and interpretation of image logs were provided by Schlumberger. Preliminary interpretation of the microseismic cluster trends reveals orientations on average of about N59°E. Image logs in the vertical pilot well reveal similar average open fracture trend of ~N58°E. The orientation of SHmax estimated from induced fractures observed in the vertical pilot well is ~N57°E, while that from breakouts is about N64°E. The majority of the induced fractures are observed in the Marcellus, while the breakouts are largely observed about 2000 feet above the Marcellus. Image logs collected along approximately 7400’ of lateral provide additional insights into the fracture network within the Marcellus target zone. Over 1600 fractures were interpreted by the Schlumberger analyst. The distribution was unimodal with average fracture trend of N78°E. Along the length of the lateral, average trends of fracture clusters varied from about N64°E to N110°E.
Shmin in the area is approximately 6500 psi with horizontal stress anisotropy varied between 100 to 400psi in agreement with the acoustic scanning platform data. The vertical stress (Sv) is approximately 8800psi. Asymmetry in the microseismicity associated with the well is interpreted to be associated with a drop in Shmin toward parallel well (the 5H well) located northeast. Hydraulic fracture stimulation of the local fracture network along the 3H well required introduction of a negative horizontal stress gradient in Shmin northeast towards the 5H well that was treated a few days earlier to produce observed asymmetry in the microseismic distribution. Variation in stage-to-stage shut-in pressures did not suggest significant stress shadowing or increase in Shmin stage-to-stage toward the heel (see Nagel et al., 2013a and b) or between wells.
In this initial look at the microseismic data, model fracture stimulation patterns are compared to microseismicity from a single stage along the 3H lateral. Initial uncalibrated MEM and stochastic based DFN models suggest that the observed microseismic event trends require interaction of the local N83°E fracture set observed in the image log along the wellbore in this area with a more regional ~N59°E fracture set. Although the inferred N59°E set is not prominent in the image log interpretations in the target landing zone, it is a prominent open fracture set in the vertical pilot well and its presence appears to control microseismic event trends and natural fracture stimulation at the site. This set appears to provide tensile conduits that channel fluids into and facilitate microseismically audible rupture of east-northeast fractures that are observed in the vicinity of the stage and that fail through shear.
Presentation Date: Tuesday, October 18, 2016
Start Time: 1:00:00 PM
Location: Lobby D/C
Presentation Type: POSTER
Meng, Xiaobo (China University of Petroleum—Beijing) | Chen, Haichao (China University of Petroleum—Beijing) | Niu, Fenglin (China University of Petroleum—Beijing) | Tang, Youcai (China University of Petroleum—Beijing) | Zuo, Qiankun (China University of Petroleum—Beijing)
We treat microseismic events with high signal to noise ratio (SNR) as master events and cross-correlate with continuous recording using both P - and S - waves to detect weak events (slave events). Subsequently, the detected weak events were located relatively to the master events, by stacking together the P - and S - waves cross - correlations along differential moveouts. Specifically, we add the azimuth restriction of P-wave to the cross - correlation stacking to improve the location accuracy in the case of single downhole acquisition. The utility of this technique is demonstrated with synthetic events. Then, we applied our method to real microseismic data from one stage of hydraulic fracturing operation.
Sharan, Shashin (University of British Columbia) | Herrmann, Felix (University of British Columbia) | Wang, Rongrong (Seismic Laboratory for Imaging and Modeling (SLIM)) | Van Leeuwen, Tristan (University of British Columbia)
In this work, we propose a new method to simultaneously locate microseismic events (e.g., induced by hydraulic fracturing) and estimate the source signature of these events. We use the method of linearized Bregman. This algorithm focuses unknown sources at their true locations by promoting sparsity along space and at the same time keeping the energy along time in check. We are particularly interested in situations where the microseismic data is noisy, sources have different signatures and we only have access to the smooth background-velocity model. We perform numerical experiments to demonstrate the usability of the proposed method. We also compare our results with full-waveform inversion based microseismic event collocation methods. Our method gives flexibility to simultaneously get a more accurate source image along with an estimate of the source-time function, which carries important information on the rupturing process and source mechanism.
Presentation Date: Tuesday, October 18, 2016
Start Time: 1:00:00 PM
Presentation Type: ORAL
Using a BEM-based hydraulic fracture model and the Coulomb frictional law, the generation of slip and shear rupturing produced by fluid injection is investigated. A wide range of factors can contribute to injection-induced seismicity. In addition to injection rate, fluid viscosity and existing permeability, several other factors are considered in this paper to determine their effects on slip development during continuous fluid injection and during shut-in. These factors include dilatant strengthening, slip weakening of frictional coefficients, structural complexities along a rupturing path like jogs and branches and the application of far-field wave-form stresses. The numerical method is summarized and the theoretical formulation of the above physical processes associated with generation of seismic events is provided. The numerical results show a rich and complex local slip rate change during extension of the slip zone. Under fluid injection, the slip can first produce a seismic event either caused by rapidly increasing pressure or by abrupt slip initiation, and then slows. When the permeability is large enough, the slip pattern manifests itself as long-term slow slip period coupled with aseismic slip transients. The long-duration and low-magnitude seismic events are associated with the slipping state in the conditionally stable regime, in which fluid pressure is marginally equal to a value required to produce slip using the effective stress and the static frictional strength. A small perturbation in the fluid pressure caused by any of the above factors can stop and/or generate relatively faster slip rates. The slip continues varying with time after shut-in in the presence of the above influencing factors. The slow slip on a fault, which is difficult to detect by seismology, could be a significant process in dissipating elastic strain energy during hydraulic fracturing stimulations. In addition, the model can capture a tendency of the slip rate to become significantly large. The results demonstrate the importance of fracture and fault permeability enhancement on variations of fluid pressure and slip rates.