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In this paper, we introduce a novel fracture imaging method which uses high resolution 3D laser scanning to develop detailed surface maps of the core fracture faces. The digital maps are then used to analyze fracture surface characteristics wherein observed variations provide us with meaningful insights into the fractures. We share a mathematical approach for roughness evaluation to identify morphological properties for individual fractures within rock samples. The approach is tested on core extracted at the Hydraulic Fracturing Test Site (HFTS - 1) in the Permian Basin. We characterize the roughness variations with depth across the cored section. In addition, we compare results obtained previously from core sampling and analysis to demonstrate that proppant entrapment observed within the cored interval is strongly correlated with the changes in fracture morphology. We also use calculated roughness along with the the changing behavior of roughness radially away from the center of fracture faces to predict roughness "types" such as propagational features or textural roughness characteristics. Based on the specific fracture characterization work shared here as well as other potential uses, our paper highlights significant advantages such scanning and digital imaging of fractures may have over traditional cataloging using photographic imaging. Furthermore, as demonstrated in this study, data sampled from these detailed maps can be used to further characterize and analyze these features in a more systematic and robust manner when compared with the more traditional geological analysis of cores.
Objectives/Scope: The Hydraulic Fracturing Test Sites (HFTS) are large collaborative field-based R&D projects funded by the US Department of Energy through the National Energy Technology Laboratory (NETL) and the E&P industry, with support from academia. The projects' main objective is to improve the understating of the hydraulic fracturing process through utilization of advanced diagnostics and collection of through-fracture cores to provide undisputable evidence and attributes of the created hydraulic fractures.
Methods/Procedures/Process: The HFTS-I is a field-based hydraulic fracturing research experiment located in the West Texas Permian (Midland) basin. At the test site, about $30 million was used to perform hydraulic fracturing research, concentrated around eleven horizontal wells fractured with over 400 stages in the upper and middle Wolfcamp formations as well as two recompleted legacy wells. Comprehensive field data was collected, including advanced diagnostics such as time-lapse cross-well seismic and microseismic surveys to measure hydraulic fracture attributes. To supplement the fracture diagnostics, two slant core wells were drilled through the created hydraulic fractures and over 850 feet of core was recovered, capturing many hundred hydraulic fractures in their natural state. The research project also completed a huff-and-puff experiment using field gas as injectant, to determine the effectiveness of such treatments for fractured shale Enhanced Oil Recovery (EOR).
Building on learnings and unanswered questions from HFTS-1, a second hydraulic fracturing research experiment (HFTS-2) has been commissioned in West Texas Permian (Delaware) basin. At the HFTS-2 eight new producing wells and two existing (legacy) wells were used to perform hydraulic fracturing research. Multiple science wells were drilled to sample and characterize the subsurface, including the collection of 540 feet of vertical core and 950 feet of high-angle through fracture core. The project also installed permanent fiber optic cables in 3 wells to monitor near wellbore signals during fracturing and to collect cross-well strain measurements. Other advanced diagnostics included a five-array microseismic survey, time-lapse geochemistry sampling and analysis, proppant log in a child well, and others.
The Hydraulic Fracturing Test Site (HFTS) Program is a research and development (R&D) partnership sponsored by the U.S Department of Energy, National Energy Technology Laboratory (DOE-NETL) and major and independent operator and service companies, managed by the Gas Technology Institute (GTI) (Ciezobka, et al. 2018, Reeves, et al. 2020). The objectives of the HFTS program are to diagnose and understand the hydraulic fracturing process for field development optimization, minimize their environmental impacts by reducing the number of new wells required for effective resource recovery, and improve extraction economics to expand the economically viable resource at increasingly lower commodity prices.
Summary We collected more than 500 ft of through‐fracture core in the Upper Wolfcamp (UWC) and Middle Wolfcamp (MWC) formations in the Permian Basin. As part of core characterization, we analyzed the core‐sludge samples for the presence of proppant and natural‐calcite particles. Apart from sample preparation and imaging, we designed and developed a novel image‐processing work flow to detect and classify the particles. We used the observations from the identified particle distribution within the stimulated rock volume to understand proppant‐transport behavior. We used relative distributions of smaller 100‐mesh‐ and larger 40/70‐mesh‐proppant particles to interpret proppant placement in relation to perforation clusters. Finally, we used the relative distribution of particles to understand the interaction between natural and hydraulic fractures. We observe that stress variations and the degree of natural fracturing have a bearing on local proppant‐screenout behavior. Smaller 100‐mesh proppant seems to dominate at larger lateral offsets from the hydraulically fractured wells. We also observe indications of heel‐side bias according to lateral proppant distribution. We share our work flow for particle detection and classification, which can serve as a template for proppant analysis in the future if significant through‐fracture cores are collected in similar field experiments.
Abstract The primary aim of this study was to develop robust methods aimed at detecting and quantifying the subsurface proppant distribution as it relates to the completed wells in both the upper and the middle Wolfcamp formations at the HFTS site. There were two sources of proppant deemed useful for this task. The first source was the actual scrapings from the fracture faces that were collected during the core description work. The second sample source was the scrapings and sludge from the cut core tubes collected during core handling. This study highlights the analysis done on the second set of sludge samples. Apart from developing a method for detection and quantification of proppant and other particles contained within this sludge, the study was aimed at the following broad objectives:Determine the spatial distribution of proppant in the created SRV along the cored interval, including size distribution and proppant concentration. Determine if pay zones of interest are sufficiently propped/ stimulated. Determine if fracture and cluster spacing is optimal for thorough lateral reservoir coverage. Determine if well spacing is optimal based on propped SRV length. Introduction A significant part of the HFTS data collection effort was the collection of substantial through fracture cores from both the upper and the middle Wolfcamp formations (UWC & MWC). In total, almost 600 ft. of core was collected using a slant core well. The location of the 11 new horizontal laterals as well as the slant core well is shown in Figure 1. The individual core barrels contain sludge from drilling, coring and core handling operations. The basic premise of our study lies in the assumption that in zones where proppant is present, a significant portion of it should show up within the core barrels. Understanding how this proppant is distributed along the cored section of the slant core well, can be instrumental in understanding fracture communication as well as propped fracture growth along the cored interval of the slant core well. Moreover, the results can then be compared with some of the other independent data available from other datasets collected as part of the HFTS program with the intention of validating or improving our understanding of said data and also helping us with our analysis. The steps followed in our analysis of the proppant are as follows:Weighing, washing, sieving, sub-sampling and various other sample preparation steps before they are imaged at high resolution using a transparency scanner. These images are then run through an automated proppant detection workflow to identify how much proppant and possible natural calcite is within the sampled material. A post picking QC step is also utilized to make sure that the final reported numbers are relatively accurate. Picked objects are segregated to classify size fractions. Post QC numbers are validated at random using direct sample observation under microscope. Two hundred and thirteen core sludge samples scraped from the interior of core barrel sleeves and exterior of the core itself were collected. This extended over all six cored sections as highlighted in Table 1. Each sample is from a 3' core tube barring a few cases where they are smaller. Table 1 displays the core section starting depths, final depths, and number of samples (tubes) recovered from each, etc. Samples varied in volume, weight and consistency. The largest sample received weighed 587g and the smallest sample weighed 7.5g. Samples contain drilling mud, shale particles, aluminum shards (from the process of milling open the core barrels to access the core), natural cement and calcite, other particles and proppant. We note that as part of this study, colored proppant was also pumped to trace the movement of proppant within the SRV. However, tests with autoclave on fresh resin coated proppant showed that under high temperature, resin itself changes color (from reddish to yellowish hue, etc.). Therefore, detection and analysis of colored proppant was not taken up as part of this study.
Abstract The Hydraulic Fracture Test Site (HFTS) is a field-based hydraulic fracturing research experiment performed in the West Texas Permian (Midland) Basin. The HFTS includes $25 million of hydraulic fracturing research that is centralized around eleven horizontal wells fractured with over 400 stages in the Upper and Middle Wolfcamp formations as well as two recompleted legacy wells. As part of the HFTS experiment, and in addition to the comprehensive field data that was collected, approximately 600 feet of excellent quality, whole core was obtained by drilling a high-angle core well through the induced hydraulic fractures at the test site. Based upon observations of the acquired core, the understanding of hydraulic fracture propagation and effectiveness of proppant placement is challenging current thinking. Well interference testing and in-situ reservoir pressure measurements using bottom hole gauges in producing wells and discrete multilevel pressure gauges in an offset monitor well aid in understating fracture connectivity and conductivity over time. Project Overview Introduction The HFTS is a field-based hydraulic fracturing research program located in the eastern part of the Midland Basin, between the Central Basin Platform and the Eastern Shelf (Figure 1, left panel). Through fostering collaboration between hydraulic fracturing experts from industry, academia and government, this research program has been established to acquire data sets to address industry-wide fundamental questions around hydraulic fracture behavior within an unconventional resource development setting. A comprehensive overview of this project is discussed in Courtier et al, 2017. Principal objectives include researching methods to reduce and minimize potential environmental impacts, demonstrate safe reliable operations, and to improve hydraulic fracturing efficiency of horizontal shale wells. Test Site The test wells are located in Reagan County, Texas, covered by a high quality 3-D seismic survey and is surrounded by many producing horizontal and vertical wells. Several adjacent wells contain open and cased-hole petrophysical, production and image logs, as well as whole cores and sidewall cores. Additionally, microseismic and tilt-meter surveys were collected during stimulation of selected wells. A total of 11 horizontal wells were drilled in the Upper and Middle Wolfcamp formations as part of this study, with five wells in the Middle Wolfcamp and six in the Upper Wolfcamp (Figure 1, right panel). The new wells are all horizontal with extended lateral sections (~ 10,000 feet), drilled from north to south, and approximately normal to the predicted maximum horizontal stress orientation. In total, over 400 hydraulic fracture stages were pumped in the test wells, of which the majority were traced and/or monitored with advanced diagnostics.