Evaluation of transmissivity in a fractured aquifer—the Nyanzari wellfield, Burundi

Due to population growth, the city of Gitega in the central part of Burundi is lacking drinking water. Therefore, the national urban water supply company decided to expand the Nyanzari wellfield by drilling additional wells.Two additional wells were drilled to 80 m (F7.2) and 85 m (F8bis) depths. Step tests followed by 72-hours aquifer tests were performed in each well. Results indicate bilinear flow followed by linear flow and radial flow in F7.2. No reaction was observed in observation wells. Fracture-matrix transmissivity was estimated at 3 · 10−4 m2/s. In the case of F8bis, linear flow in an infinite flow fracture followed by radial flow was visible. Reaction was measured in observation wells. Transmissivity was estimated at 3.3 · 10−3 m2/s.Both wells lie no more than 300 m apart, but no evidence of interference between them was depicted during the tests. It appears that two independent fracture systems prevail in the wellfield.

Facile and Controllable Preparation of poly(St-co-MMA)/FA Microspheres Used as Ultra-Lightweight Proppants

Hydraulic fracturing is an important technology for the exploitation of unconventional oil or gas reservoirs. In order to increase the production of oil or gas, ultra-lightweight proppants with a high compressive strength are highly desirable in hydraulic fracture systems. In this work, a new type of ultra-lightweight proppant, poly(styrene-co-methyl methacrylate)/fly ash (poly(St-co-MMA)/FA) composites with a high compressive strength were prepared via in situ suspension polymerization. The Fourier transform infrared (IR) and X-ray powder diffraction (XRD) analyses confirmed that the poly(St-co-MMA)/FA composites were successfully prepared. The morphology analysis indicated that the composite microspheres show good sphericity, and FA powder was evenly dispersed in the matrix. The apparent density of the microspheres was between 1 and 1.3 g/cm3, which is suitable for hydraulic fracturing. Furthermore, the compressive strength and thermostability were dramatically improved with the incorporation of FA, which could withstand high pressures and temperatures underground. The obtained poly(St-co-MMA)/FA composite microspheres are promising for application as an ultra-lightweight (ULW) proppant in oil or gas exploitation, which provides a new approach for the design of high performance proppants.

Fracture system characterisation and implications for fluid flow in volcanic and metamorphic rocks

<p>Fluid flow pathways in volcanic and metamorphic rocks are dominantly controlled by fracture systems. Although these fracture systems are critical for developing reservoirs in an economical and sustainable way, and for understanding processes that cause earthquakes, they are often poorly constrained. This thesis studies the geometry of fracture systems, the factors influencing their geometries, and their possible impacts on permeability in three contrasting settings: an outcropping andesite lava flow of the Ruapehu volcano; the andesite-hosted Rotokawa geothermal reservoir; and the Alpine Fault hangingwall metamorphosed schists. We use datasets from a combination of cores, acoustic borehole televiewer (BHTV) logs, outcrop scanlines, and terrestrial laser scanner (TLS) point clouds, which span multiple scales of observation. Fracture geometries are studied in a young (~6 ka-old) blocky andesitic lava flow on the Ruapehu volcano, as a representative example of weakly-altered andesitic lava flows emplaced over gentle topography in the absence of glaciers. Fractures were formed during cooling and emplacement of the lava flow. Fractures are automatically detected from the 3-D TLS point cloud of an outcrop area of ~3090 m2 using a plane detection algorithm, and calibrated with manual scanlines and high-resolution panoramic photographs. Column-forming fractures dominate the fracture system, are either sub-horizontal or sub-vertical (i.e., sub-parallel or sub-perpendicular to the brecciated margins) without mean strike orientation, and have an exponential length distribution. Sub-horizontal, clustered platy fractures sub-parallel to the flow direction arrest or deflect column-forming fractures. Areal and volumetric fracture intensity analyses reveal a ~0.5 % connected fracture volume which, although seemingly small, promotes fluid flow due to the planarity and connectivity of the system. Autobreccias are partially connected to column-forming fractures, and may promote lateral flow or form barriers depending on the extent of post-cooling alteration and mineralisation. Discrete fracture network models generated with the measured geometrical parameters are in agreement with the observed highly connected fracture system. Fractures in the andesite-hosted Rotokawa Geothermal Field are described in cores and BHTV logs. Fractures interpreted on BHTV logs are separated into sets of similar orientation using quantifiable clustering algorithms. Fracture thickness and spacing probability distributions are estimated from maximum likelihood estimations applied to truncated distributions, taking sampling biases into consideration. Spacing of the predominant sub-vertical NE-SW-striking fracture set, and subordinate NW-SE-striking fracture set, are best approximated by log-normal distributions and interpreted to be controlled by stratifications within the lava flow sequence. By contrast, spacing of other subordinate fracture sets, either dipping 60° and striking NE-SW, or steeply dipping and striking N-S, are best approximated by power-law distributions and interpreted to be fault-controlled. Fracture thicknesses in both cores and BHTV logs are approximated by a single power-law distribution, which reflects heterogeneous pathways observed at reservoir scale. Previously reported ~5 µm-thick fractures studied in thin section do not follow this power-law distribution and have an isotropic orientation, which suggests a change of controls on fracture density and orientation from thermal stresses at thin-section scale, to tectonic and lithological at core and BHTV log scales. However, fractures occupy ~5 % of the rock mass at the three scales of observations, suggesting a self-similar behaviour of fracture volumes in 3-D. In contrast to the Ruapehu and Rotokawa reservoir studies, scientific drilling in 2014 of the DFDP-2B borehole offered a unique opportunity to investigate the foliation and fractures along a 630 m-long borehole section in metamorphic rocks in the hangingwall of the Alpine Fault. BHTV log interpretation reveals a constant foliation and foliation-parallel fracture orientation (60°/145°; dip magnitude/dip direction) similar to nearby outcrops and parallel to the regional strike of the Alpine Fault. This foliation orientation may reflect the orientation of the Alpine Fault at ~1 km depth. In addition, sub-vertical fractures striking NW-SE above ~500 m, and sub-horizontal fractures between ~ 500-820 m below ground, are interpreted as exhumation-related joints and inherited hydrofractures respectively. Finally, we recognise metre-thick fault zones similar to those identified from BHTV logs and cores in the nearby DFDP-1B borehole. The three fracture set orientations, and observed fault zones, promote high hydraulic connectivity in the Alpine Fault hangingwall, which fosters fluid flow. This thesis helps quantify the geometrical parameters of fractures and their associated uncertainties in non-sedimentary settings, which are required to constrain numerical models and unravel fluid flow pathways in heterogeneous rocks. We identified lithological, tectonic and thermal controls on fracture geometries, which can constrain conditions and processes by which these fractures formed, and improve the prediction of fracture system architecture away from sparse borehole observations. The results of this thesis are relevant to similar lithological and tectonic settings elsewhere where observations are scarce. This study has also yielded an essential fracture dataset for better understanding of the structural and hydrological conditions at depth near the Alpine Fault prior to a large earthquake.</p>

Fracture system characterisation and implications for fluid flow in volcanic and metamorphic rocks

<p>Fluid flow pathways in volcanic and metamorphic rocks are dominantly controlled by fracture systems. Although these fracture systems are critical for developing reservoirs in an economical and sustainable way, and for understanding processes that cause earthquakes, they are often poorly constrained. This thesis studies the geometry of fracture systems, the factors influencing their geometries, and their possible impacts on permeability in three contrasting settings: an outcropping andesite lava flow of the Ruapehu volcano; the andesite-hosted Rotokawa geothermal reservoir; and the Alpine Fault hangingwall metamorphosed schists. We use datasets from a combination of cores, acoustic borehole televiewer (BHTV) logs, outcrop scanlines, and terrestrial laser scanner (TLS) point clouds, which span multiple scales of observation. Fracture geometries are studied in a young (~6 ka-old) blocky andesitic lava flow on the Ruapehu volcano, as a representative example of weakly-altered andesitic lava flows emplaced over gentle topography in the absence of glaciers. Fractures were formed during cooling and emplacement of the lava flow. Fractures are automatically detected from the 3-D TLS point cloud of an outcrop area of ~3090 m2 using a plane detection algorithm, and calibrated with manual scanlines and high-resolution panoramic photographs. Column-forming fractures dominate the fracture system, are either sub-horizontal or sub-vertical (i.e., sub-parallel or sub-perpendicular to the brecciated margins) without mean strike orientation, and have an exponential length distribution. Sub-horizontal, clustered platy fractures sub-parallel to the flow direction arrest or deflect column-forming fractures. Areal and volumetric fracture intensity analyses reveal a ~0.5 % connected fracture volume which, although seemingly small, promotes fluid flow due to the planarity and connectivity of the system. Autobreccias are partially connected to column-forming fractures, and may promote lateral flow or form barriers depending on the extent of post-cooling alteration and mineralisation. Discrete fracture network models generated with the measured geometrical parameters are in agreement with the observed highly connected fracture system. Fractures in the andesite-hosted Rotokawa Geothermal Field are described in cores and BHTV logs. Fractures interpreted on BHTV logs are separated into sets of similar orientation using quantifiable clustering algorithms. Fracture thickness and spacing probability distributions are estimated from maximum likelihood estimations applied to truncated distributions, taking sampling biases into consideration. Spacing of the predominant sub-vertical NE-SW-striking fracture set, and subordinate NW-SE-striking fracture set, are best approximated by log-normal distributions and interpreted to be controlled by stratifications within the lava flow sequence. By contrast, spacing of other subordinate fracture sets, either dipping 60° and striking NE-SW, or steeply dipping and striking N-S, are best approximated by power-law distributions and interpreted to be fault-controlled. Fracture thicknesses in both cores and BHTV logs are approximated by a single power-law distribution, which reflects heterogeneous pathways observed at reservoir scale. Previously reported ~5 µm-thick fractures studied in thin section do not follow this power-law distribution and have an isotropic orientation, which suggests a change of controls on fracture density and orientation from thermal stresses at thin-section scale, to tectonic and lithological at core and BHTV log scales. However, fractures occupy ~5 % of the rock mass at the three scales of observations, suggesting a self-similar behaviour of fracture volumes in 3-D. In contrast to the Ruapehu and Rotokawa reservoir studies, scientific drilling in 2014 of the DFDP-2B borehole offered a unique opportunity to investigate the foliation and fractures along a 630 m-long borehole section in metamorphic rocks in the hangingwall of the Alpine Fault. BHTV log interpretation reveals a constant foliation and foliation-parallel fracture orientation (60°/145°; dip magnitude/dip direction) similar to nearby outcrops and parallel to the regional strike of the Alpine Fault. This foliation orientation may reflect the orientation of the Alpine Fault at ~1 km depth. In addition, sub-vertical fractures striking NW-SE above ~500 m, and sub-horizontal fractures between ~ 500-820 m below ground, are interpreted as exhumation-related joints and inherited hydrofractures respectively. Finally, we recognise metre-thick fault zones similar to those identified from BHTV logs and cores in the nearby DFDP-1B borehole. The three fracture set orientations, and observed fault zones, promote high hydraulic connectivity in the Alpine Fault hangingwall, which fosters fluid flow. This thesis helps quantify the geometrical parameters of fractures and their associated uncertainties in non-sedimentary settings, which are required to constrain numerical models and unravel fluid flow pathways in heterogeneous rocks. We identified lithological, tectonic and thermal controls on fracture geometries, which can constrain conditions and processes by which these fractures formed, and improve the prediction of fracture system architecture away from sparse borehole observations. The results of this thesis are relevant to similar lithological and tectonic settings elsewhere where observations are scarce. This study has also yielded an essential fracture dataset for better understanding of the structural and hydrological conditions at depth near the Alpine Fault prior to a large earthquake.</p>

Technology Focus: Hydraulic Fracturing Modeling (November 2021)

In the past decades, the success of unconventional hydrocarbon resource development can be attributed primarily to the improved understanding of fracture systems, including both hydraulically induced fractures and natural fracture networks. To tackle the fracture characterization problem, several recent papers have provided novel insights into fracture modeling technique. Because of the complex nature and heterogeneity of rock discontinuity, fabric, and texture, the fracture-modeling process typically suffers from limited data availability. Research shows that modeling results reached without interrogation of high-resolution petrophysical and geomechanical data can mislead because the fluid flow is actually controlled by fine-scale rock properties. A more-reliable fracture geometry can be obtained from an enhanced modeling process that preserves the signature from high-frequency data. Advanced techniques to model fracturing processes with proppant transportation and thermodynamics require even more-sophisticated simulation and computation power. When the subsurface is too puzzling to be described by a physical model and existing data, machine learning and artificial intelligence can be adapted as a practical alternative to interpret complex fracture systems. Taking a discrete fracture network (DFN) as an example, a data-driven approach has been introduced to learn from outcrop, borehole imaging, core computed tomography scan, and seismic data to recognize stratigraphic bedding, faults, subseismic fractures, and hydraulic fractures. Input data can be collected by hand, 3D stereophotogrammetry, or drone. When upscaling DFN into a coarse grid for reservoir simulation, deep-learning techniques such as convolutional neuron networks can be used to populate fracture properties into a dual-porosity/dual-permeability model approved to yield high accuracy compared with a fine-grid model. Furthermore, the new techniques greatly extend the application of fracture modeling in the arena of the energy transition, such as in geothermal optimization. Recommended additional reading at OnePetro: www.onepetro.org. SPE 203927 - Numerical Simulation of Proppant Transport in Hydraulically Fractured Reservoirs by Seyhan Emre Gorucu, Computer Modelling Group, et al. SPE 202679 - Deep-Learning Approach To Predict Rheological Behavior of Supercritical CO2 Foam Fracturing Fluid Under Different Operating Conditions by Shehzad Ahmed, Khalifa University of Science and Technology, et al. SPE 203983 - A 3D Coupled Thermal/Hydraulic/Mechanical Model Using EDFM and XFEM for Hydraulic-Fracture-Dominated Geothermal Reservoirs by Xiangyu Yu, Colorado School of Mines, et al.

Direct Visualization and Quantification of NCF-Strengthened CO2 Foam Generation, Propagation and Sweep in a 2D Heterogeneous Fracture Network Model

CO2 foam holds promising potential for conformance improvement and mobility reduction of CO2 injection in fractured systems. However, there still exists two main issues hampering its wide application and development, 1. Instability of CO2 foam lamellae under reservoir conditions, and 2. Uncertainties of foam flow in fracture systems. To address these two issues, we previously developed a series of functional nanocellulose materials to stabilize the CO2 foam (referred to NCF-st-CO2 foam), while the primary goal of this paper is to thoroughly elucidate foam generation, propagation and sweep of NCF-st-CO2 foam in fractured systems by using a self-designed visual heterogeneous fracture network. We found that NCF-st-CO2 foam produced noticeably greater pressure drop (ΔP) than CO2 foam during either co-injection (COI) or surfactant solution-alternating-gas (SAG) injection, and the threshold foam quality (fg*) was approximately 0.67. Foam generation was increased with total flow rate for CO2 foam and stayed constant for NCF-st-CO2 foam in fracture during COI. CO2 breakthrough occurred at high flow rates (&gt;8 cm3/min). For SAG, large surfactant slug could prevent CO2 from early breakthrough and facilitate foaming in-situ. The increase in sweep efficiency by NCF-st-CO2 foam was observed near the producer for both COI and WAG, which was attributed to its better foaming capacity. Film division and behind mainly led to foam generation in the fracture model. Gravity segregation and override was insignificant during COI but became noticeable during SAG, which caused the sweep efficiency decreased by 3~9% at 1.0 fracture volume (FV) injected. Due to the enhanced foam film, the NCF-st-CO2 foam was able to mitigate gravitational effect, especially in the vicinity of producer.

Review: Saltwater intrusion in fractured crystalline bedrock

During the past few years, the number of regional and national assessments of groundwater quality in regard to saltwater intrusion in coastal aquifers has increased steadily. However, most of the international literature on saltwater intrusion is focused on coastal plains with aquifers in unconsolidated material. Case studies, modelling approaches and parameter studies dealing with saltwater intrusion in those systems are abundant. While the hydrogeology of fractured rock has been intensively studied with both modelling approaches and parameter studies—mainly in relation to deep-laying fractured crystalline bedrock as potential waste repositories—case studies on saltwater intrusion in shallow fractured rocks are still an exception. This review summarizes the actual knowledge on saltwater intrusion in fractured crystalline rock. In combination with short overviews of the processes of saltwater intrusion, flow in fractured systems and the genesis of these systems, the review highlights the importance of the fracture systems and its specific characteristics. Fracture properties are a direct consequence of the geological history as well as the current situation of the coastal area. A holistic assessment of water quality in coastal areas hosting fractured crystalline bedrock therefore requires the combination of different approaches in order to investigate the impact of saltwater intrusion through the fractured system.

Nested Recharge Systems in Mountain Block Hydrology: High-Elevation Snowpack Generates Low-Elevation Overwinter Baseflow in a Rocky Mountain River

The majority of each year′s overwinter baseflow (i.e., winter streamflow) in a third-order eastern slopes tributary is generated from annual melting of high-elevation snowpack which is transmitted through carbonate and siliciclastic aquifers. The Little Elbow River and its tributaries drain a bedrock system formed by repeated thrust faults that express as the same siliciclastic and carbonate aquifers in repeating outcrops. Longitudinal sampling over an 18 km reach was conducted at the beginning of the overwinter baseflow season to assess streamflow provenance. Baseflow contributions from each of the two primary aquifer types were apportioned using sulfate, δ34SSO4, and silica concentrations, while δ18OH2O composition was used to evaluate relative temperature and/or elevation of the original precipitation. Baseflow in the upper reaches of the Little Elbow was generated from lower-elevation and/or warmer precipitation primarily stored in siliciclastic units. Counterintuitively, baseflow generated in the lower-elevation reaches originated from higher-elevation and/or colder precipitation stored in carbonate units. These findings illustrate the role of nested flow systems in mountain block recharge: higher-elevation snowmelt infiltrates through fracture systems in the cliff-forming—often higher-elevation—carbonates, moving to the lower-elevation valley through intermediate flow systems, while winter baseflow in local flow systems in the siliciclastic valleys reflects more influence from warmer precipitation. The relatively fast climatic warming of higher elevations may alter snowmelt timing, leaving winter water supply vulnerable to climatic change.