Sequential Stimulation Research Articles

Stimulation of abdominal and upper thoracic muscles with surface electrodes for respiration and cough: Acute studies in adult canines

Objective: To optimize maximal respiratory responses with surface stimulation over abdominal and upper thorax muscles and using a 12-Channel Neuroprosthetic Platform. Methods: Following instrumentation, six anesthetized adult canines were hyperventilated sufficiently to produce respiratory apnea. Six abdominal tests optimized electrode arrangements and stimulation parameters using bipolar sets of 4.5 cm square electrodes. Tests in the upper thorax optimized electrode locations, and forelimb moment was limited to slight-to-moderate. During combined muscle stimulation tests, the upper thoracic was followed immediately by abdominal stimulation. Finally, a model of glottal closure for cough was conducted with the goal of increased peak expiratory flow. Results: Optimized stimulation of abdominal muscles included three sets of bilateral surface electrodes located 4.5 cm dorsal to the lateral line and from the 8th intercostal space to caudal to the 13th rib, 80 or 100 mA current, and 50 Hz stimulation frequency. The maximal expired volume was 343 ± 23 ml (n=3). Optimized upper thorax stimulation included a single bilateral set of electrodes located over the 2nd interspace, 60 to 80 mA, and 50 Hz. The maximal inspired volume was 304 ± 54 ml (n=4). Sequential stimulation of the two muscles increased the volume to 600 ± 152 ml (n=2), and the glottal closure maneuver increased the flow. Conclusions: Studies in an adult canine model identified optimal surface stimulation methods for upper thorax and abdominal muscles to induce sufficient volumes for ventilation and cough. Further study with this neuroprosthetic platform is warranted.

Fc Gamma Receptor IIB Deficient Mice: A Lupus Model with Increased Endotoxin Tolerance-Related Sepsis Susceptibility.

Hyper-elevated immune response of FcGRIIb-/- mice, a lupus model with an inhibitory-signaling defect, can become exhausted (less subsequent immune-response than the first response) with sequential lipopolysaccharide (LPS) stimulation. Endotoxin tolerance-related modifications of inflammatory response were investigated in FcGRIIb-/- mice in both an in vivo sepsis model and in vitro using cultured macrophages. Serum cytokine concentrations, after the second LPS injection (at 5-fold higher levels than the first dose), did not exceed the first dose levels in either FcGRIIb-/- or wild-type mice. These data indicated an endotoxin-tolerance response in both genetic backgrounds. However, the difference of cytokine levels between the first and second LPS injection was more prominent in FcGRIIb-/- mice. More importantly, CLP-induced sepsis after LPS-preconditioning (two separated doses of LPS administration) was more severe in FcGRIIb-/- mice (as measured by mortality rate, bacteria count in blood, serum cytokines, creatinine, and alanine transaminase). An attenuated response was demonstrated after two sequential LPS stimulations of bone-marrow-derived macrophages. Cytokine production was reduced and lower bacterial killing activity occurred with macrophages from FcGRIIb-/- mice relative to wild-type macrophages. Thus, there is a more prominent effect of endotoxin-tolerance in FcGRIIb-/- macrophages relative to wild-type. In conclusion, repeated-LPS administrations induced quantitatively greater endotoxin-tolerance responses in FcGRIIb-/- mice both in vivo and in vitro. Endotoxin-tolerance in vivo was associated with more severe sepsis, at least in part, due to macrophage-dysfunction.

Optogenetic Stimulation of Cortex to Map Evoked Whisker Movements in Awake Head-Restrained Mice

Whisker movements are used by rodents to touch objects in order to extract spatial and textural tactile information about their immediate surroundings. To understand the mechanisms of such active sensorimotor processing it is important to investigate whisker motor control. The activity of neurons in the neocortex affects whisker movements, but many aspects of the organization of cortical whisker motor control remain unknown. Here, we filmed whisker movements evoked by sequential optogenetic stimulation of different locations across the left dorsal sensorimotor cortex of awake head-restrained mice. Whisker movements were evoked by optogenetic stimulation of many regions in the dorsal sensorimotor cortex. Optogenetic stimulation of whisker sensory barrel cortex evoked retraction of the contralateral whisker after a short latency, and a delayed rhythmic protraction of the ipsilateral whisker. Optogenetic stimulation of frontal cortex evoked rhythmic bilateral whisker protraction with a longer latency compared to stimulation of sensory cortex. Compared to frontal cortex stimulation, larger amplitude bilateral rhythmic whisking in a less protracted position was evoked at a similar latency by stimulating a cortical region posterior to Bregma and close to the midline. These data suggest that whisker motor control might be broadly distributed across the dorsal mouse sensorimotor cortex. Future experiments must investigate the complex neuronal circuits connecting specific cell-types in various cortical regions with the whisker motor neurons located in the facial nucleus.

P2X7 Deficiency Blocks Lesional Inflammasome Activity and Ameliorates Atherosclerosis in Mice.

Extracellular adenosine triphosphate (ATP) binds as a danger signal to purinergic receptor P2X7 and promotes inflammasome assembly and interleukin-1β expression. We hypothesized a functional role of the signal axis ATP-P2X7 in inflammasome activation and the chronic inflammation driving atherosclerosis. P2X7-competent and P2X7-deficient macrophages were isolated and stimulated with lipopolysaccharide, ATP, or both. To assess whether P2X7 may have a role in atherosclerosis, P2X7 expression was analyzed in aortic arches from low density lipoprotein receptor-/- mice consuming a high-cholesterol or chow diet. P2X7+/+ and P2X7-/- low density lipoprotein receptor-/- mice were fed a high-cholesterol diet to investigate the functional role of P2X7 knockout in atherosclerosis. Human plaques were derived from carotid endarterectomy and stained against P2X7. Lipopolysaccharide or ATP stimulation alone did not activate caspase 1 in isolated macrophages. However, priming with lipopolysaccharide, followed by stimulation with ATP, led to an activation of caspase 1 and interleukin-1β in P2X7-competent macrophages. In contrast, P2X7-deficient macrophages showed no activation of caspase 1 after sequential stimulation while still expressing a basal amount of interleukin-1β. P2X7 receptor was higher expressed in murine atherosclerotic lesions, particularly by lesional macrophages. After 16 weeks of a high-cholesterol diet, P2X7-deficient mice showed smaller atherosclerotic lesions than P2X7-competent mice (0.162 cm2±0.023 [n=9], P2X7-/- low density lipoprotein receptor-/- : 0.084 cm2±0.01 [n=11], P=0.004) with a reduced amount of lesional macrophages. In accord with our in vitro findings, lesional caspase 1 activity was abolished in P2X7-/- mice. In addition, intravital microscopy revealed reduced leukocyte rolling and adhesion in P2X7-deficient mice. Last, we observe increased P2X7 expression in human atherosclerotic lesions, suggesting that our findings in mice are relevant for human disease. P2X7 deficiency resolved plaque inflammation by inhibition of lesional inflammasome activation and reduced experimental atherosclerosis. Therefore, P2X7 represents an interesting potential new target to combat atherosclerosis.

Additive effects of sequential excitatory and inhibitory theta burst stimulation in improving cortical excitability following ischaemic stroke

ABSTRACTPurpose: To study the effects of sequential excitatory and inhibitory theta burst stimulation (TBS) on cortical excitability in patients with subacute ischaemic stroke.Methods: Fifteen patients with first-ever middle cerebral artery ischaemic stroke within last three months were subjected to intermittent TBS over the hand area (M1) of the stroke hemisphere (iTBSSH) followed by continuous TBS over the intact hemisphere M1 (cTBSIH). The study measured motor evoked potential (MEP) amplitude, resting motor threshold (RMT) and contralateral cortical silent period (cCSP) at baseline and after 7 minutes of each TBS session.Results: The mean age of the patients was 62.4 ± 8.0 years. The absolute changes in baseline motor physiological parameters were significantly greater after cTBSIH than after iTBSSH (p = 0.001). Following iTBSSH, there was a significant decrease in mean RMT (7.4%; p < 0.001). Following cTBSIH, there was further 18.3% increase in mean MEP amplitude and decrease in mean RMT (4.7%; p < 0.001) and cCSP (5.6%; p < 0.001) of the stroke hemisphere. Similar changes of smaller magnitude were noted in 12 age-matched controls.Conclusions: The results indicate that combining iTBSSH and cTBSIH can have additive effect in increasing cortical excitability in subacute ischaemic patients with stroke.

Bilateral sequential motor cortex stimulation and skilled task performance with non-dominant hand.

To check whether bilateral sequential stimulation (BSS) of M1 with theta burst stimulation (TBS), using facilitatory protocol over non-dominant M1 followed by inhibitory one over dominant M1, can improve skilled task performance with non-dominant hand more than either of the unilateral stimulations do. Both, direct motor cortex (M1) facilitatory non-invasive brain stimulation (NIBS) and contralateral M1 inhibitory NIBS were shown to improve motor learning. Forty right-handed healthy subjects were divided into 4 matched groups which received either ipsilateral facilitatory (intermittent TBS [iTBS] over non-dominant M1), contralateral inhibitory (continuous TBS [cTBS] over dominant M1), bilateral sequential (contralateral cTBS followed by ipsilateral iTBS), or placebo stimulation. Performance was evaluated by Purdue peg-board test (PPT), before (T0), immediately after (T1), and 30min after (T2) an intervention. In all groups and for both hands, the PPT scores increased at T1 and T2 in comparison to T0, showing clear learning effect. However, for the target non-dominant hand only, immediately after BSS (at T1) the PPT scores improved significantly more than after either of unilateral interventions or placebo. M1 BSS TBS is an effective intervention for improving motor performance. M1 BSS TBS seems as a promising tool for motor learning improvement with potential uses in neurorehabilitation.

Sequential Theta burst stimulation changes language function after stroke - Preliminary analysis in Chinese survivors

Sequential Theta burst stimulation changes language function after stroke - Preliminary analysis in Chinese survivors

Vaccination with Altered Peptide Ligands of a Plasmodium berghei Circumsporozoite Protein CD8 T-Cell Epitope: A Model to Generate T Cells Resistant to Immune Interference by Polymorphic Epitopes.

Many pathogens, including the malaria parasite Plasmodium falciparum, display high levels of polymorphism within T-cell epitope regions of proteins associated with protective immunity. The T-cell epitope variants are often non-cross-reactive. Herein, we show in a murine model, which modifies a protective CD8 T-cell epitope from the circumsporozoite protein (CS) of Plasmodium berghei (SYIPSAEKI), that simultaneous or sequential co-stimulation with two of its putative similarly non-cross-reactive altered peptide ligand (APL) epitopes (SYIPSAEDI or SYIPSAEAI) has radically different effects on immunity. Hence, co-immunization or sequential stimulation in vivo of SYIPSAEKI with its APL antagonist SYIPSAEDI decreases immunity to both epitopes. By contrast, co-immunization with SYIPSAEAI has no apparent initial effect, but it renders the immune response to SYIPSAEKI resistant to being turned off by subsequent immunization with SYIPSAEDI. These results suggest a novel strategy for vaccines that target polymorphic epitopes potentially capable of mutual immune interference in the field, by initiating an immune response by co-immunization with the desired index epitope, together with a carefully selected “potentiator” APL peptide.

Reducing interaction in simultaneous paired stimulation with CI.

In this study simultaneous paired stimulation of electrodes in cochlear implants is investigated by psychophysical experiments in 8 post-lingually deaf subjects (and one extra subject who only participated in part of the experiments). Simultaneous and sequential monopolar stimulation modes are used as references and are compared to channel interaction compensation, partial tripolar stimulation and a novel sequential stimulation strategy named phased array compensation. Psychophysical experiments are performed to investigate both the loudness integration during paired stimulation at the main electrodes as well as the interaction with the electrode contact located halfway between the stimulating pair. The study shows that simultaneous monopolar stimulation has more loudness integration on the main electrodes and more interaction in between the electrodes than sequential stimulation. Channel interaction compensation works to reduce the loudness integration at the main electrodes, but does not reduce the interaction in between the electrodes caused by paired stimulation. Partial tripolar stimulation uses much more current to reach the needed loudness, but shows the same interaction in between the electrodes as sequential monopolar stimulation. In phased array compensation we have used the individual impedance matrix of each subject to calculate the current needed on each electrode to exactly match the stimulation voltage along the array to that of sequential stimulation. The results show that the interaction in between the electrodes is the same as monopolar stimulation. The strategy uses less current than partial tripolar stimulation, but more than monopolar stimulation. In conclusion, the paper shows that paired stimulation is possible if the interaction is compensated.

Tissue mechanics during temporal summation of sequentially cuff pressure-induced pain in healthy volunteers and patients with painful osteoarthritis.

The phenomenon of temporal summation to repetitive pressure pain stimuli is an important central neural mechanism for pain intensity encoding. This study evaluated the time-dependent behaviour of mechanical characteristics of soft tissue during repeated cuff stimulation used for eliciting temporal summation of cuff pressure-evoked pain. Such information of tissue mechanics is important for the interpretation of the pain response evoked during sequential stimulations. Temporal summation was assessed in 16 subjects separated into two groups (healthy controls and severe knee osteoarthritis patients) using a visual analogue scale during 10 repetitive painful cuff stimuli (1-s duration, 1-s break) of the lower leg. The geometry of the lower leg was constructed based on magnetic resonance image (MRI) data. The loading boundary condition of the finite element model was defined according to the parabolic pattern of the interface pressure around the limb and the time-dependent profile of the cuff pressure during repetitive stimuli. The pain intensity significantly increased with an increasing number of stimuli (p<0.001), and facilitated temporal summation of pain was observed in patients compared with healthy controls (p<0.001). The maximal deep tissue stress and strain during stimuli 1-4 varied 43% and 9%, respectively. No variation was observed for stimuli 5-10. The study concludes that the temporal summation of pain response during sequential cuff pressure is not explicable by a specific time-dependent behaviour of stress and strain in the activated deep tissue and hence not due to changes in tissue biomechanics. The temporal summation of pain during sequential cuff stimulation is inexplicable by the time-dependent response of mechanical stress and strain in soft tissue.

Adding simultaneous stimulating channels to reduce power consumption in cochlear implants

Sound coding strategies for Cochlear Implant (CI) listeners can be used to control the trade-off between speech performance and power consumption. Most commercial CI strategies use non-simultaneous channel stimulation, stimulating only one electrode at a time. One could add parallel simultaneous stimulating channels such that the electrical interaction between channels is increased. This would produce spectral smearing, because the electrical fields of the simultaneous stimulated channels interact, but also power savings. The parallel channels produce a louder sensation than sequential stimulation. To test this hypothesis we implemented different sound coding strategies using a research interface from Advanced Bionics: the commercial F120 strategy using sequential channel stimulation (one channel equals two electrodes with current steering) and the Paired strategy, consisting of simultaneous stimulation with two channels. Here, the electrical field of both channels will interact, requiring less current on each channel to perceive the same loudness as with F120. However, channel interaction between the independent channels may reduce speech recognition or understanding. This can be diminished by adding an inverse-polarity stimulation channel between both channels. This strategy is termed Paired with Flanks. Additionally, Triplet with three channels and an adjacent Flank style was investigated. For each strategy we measured speech intelligibility with the Hochmair-Schulz-Moser sentence test. Spectral resolution was assessed using a spectral modulation depth detection task. Results show that Paired without Flanks obtains similar performance while reducing the current by 20% on average compared to F120. Triplet with and without Flanks shows overall poorer performance when compared to F120. All strategies inhibit the option to increase the pulse width which would result in even further decreased power consumption.

Production analysis of sequential multi-well cyclic steam stimulation in heterogeneous heavy oil reservoir

Sequential multi-well cyclic steam stimulation (CSS) performs better than conventional CSS in heavy oil reservoirs. It distributes all wells into two or more groups. Each group injects steam simultaneously while different groups inject one by one. This paper further studies the effect of heterogeneity on well grouping in sequential multi-well CSS and explains how it improves conventional CSS. Results indicate that the channelling wells should be put into a same group and injected simultaneously. This can expand heating area, increase moveable oil and replenish reservoir energy. After appropriate well grouping, injecting group by group can establish a pressure drop between injecting and producing groups. The pressure drop and oil displacement directions change periodically. These mechanisms improve the performance of sequential multi-well CSS in heavy oil reservoirs on both technical and economic sides. [Received: March 25, 2016; Accepted: July 24, 2016]

Production analysis of sequential multi-well cyclic steam stimulation in heterogeneous heavy oil reservoir

Sequential multi-well cyclic steam stimulation (CSS) performs better than conventional CSS in heavy oil reservoirs. It distributes all wells into two or more groups. Each group injects steam simultaneously while different groups inject one by one. This paper further studies the effect of heterogeneity on well grouping in sequential multi-well CSS and explains how it improves conventional CSS. Results indicate that the channelling wells should be put into a same group and injected simultaneously. This can expand heating area, increase moveable oil and replenish reservoir energy. After appropriate well grouping, injecting group by group can establish a pressure drop between injecting and producing groups. The pressure drop and oil displacement directions change periodically. These mechanisms improve the performance of sequential multi-well CSS in heavy oil reservoirs on both technical and economic sides. [Received: March 25, 2016; Accepted: July 24, 2016]

3-D Geostatistical Model and Volumetric Estimation of ‘Del’ Field, Niger Delta

There is an insatiable thirst for oil and gas consumption and increased production will be made possible only through effective reservoir characterization and modeling. A suite of wire-line logs for four wells from ‘DEL’ oil field together with 3D seismic data were analyzed for reservoir characterization of the field. Two reservoirs were identified using the resistivity log. A synthetic seismogram was generated in order to perform seismic to well tie process as well as picking of horizons throughout the section. Time and depth structural maps were generated. Geostatistical simulation such as the sequential Gaussian stimulation and sequential indicator stimulation were carried out to provide equiprobable representations of the reservoirs, and the distribution of reservoir properties within the geological cells. The modeled reservoir properties resulted in an improved description of reservoir distribution and inter connectivity. The analysis indicated the presence of hydrocarbon in the reservoirs. There is also a fault assisted closure on the structural map which is of interest in exploration. A fluid distribution plot and map of the field were also obtained. The modeled properties gave an average porosity of 24%, average water saturation ranging from 12%-24% and moderate net-gross. The volumetric calculation of the reservoir gives a STOIIP ranging from 37.53 MMbbl-43.03 MMbbl. The result showed high hydrocarbon potential and a reservoir system whose performance is considered satisfactory for hydrocarbon production. The resulting models can also be used to predict the future performance of the reservoir.

A novel ATP-synthase-independent mechanism coupling mitochondrial activation to exocytosis in insulin-secreting cells.

Pancreatic β-cells sense glucose, promoting insulin secretion. Glucose sensing requires the sequential stimulation of glycolysis, mitochondrial metabolism and Ca2+ entry. To elucidate how mitochondrial activation in β-cells contributes to insulin secretion, we compared the effects of glucose and the mitochondrial substrate methylsuccinate in the INS-1E insulin-secreting cell line at the respective concentrations at which they maximally activate mitochondrial respiration. Both substrates induced insulin secretion with distinct respiratory profiles, mitochondrial hyperpolarization, NADH production and ATP-to-ADP ratios. In contrast to glucose, methylsuccinate failed to induce large [Ca2+] rises and exocytosis proceeded largely independently of mitochondrial ATP synthesis. Both glucose- and methylsuccinate-induced secretion was blocked by diazoxide, indicating that Ca2+ is required for exocytosis. Dynamic assessment of the redox state of mitochondrial thiols revealed a less marked reduction in response to methylsuccinate than with glucose. Our results demonstrate that insulin exocytosis can be promoted by two distinct mechanisms one of which is dependent on mitochondrial ATP synthesis and large Ca2+ transients, and one of which is independent of mitochondrial ATP synthesis and relies on small Ca2+ signals. We propose that the combined effects of Ca2+ and redox reactions can trigger insulin secretion by these two mechanisms.

Fatigue reduction during aggregated and distributed sequential stimulation

Transcutaneous neuromuscular electrical stimulation (NMES) can generate muscle contractions for rehabilitation and exercise. However, NMES-evoked contractions are limited by fatigue when they are delivered "conventionally" (CONV) using a single active electrode. Researchers have developed "sequential" (SEQ) stimulation, involving rotation of pulses between multiple "aggregated" (AGGR-SEQ) or "distributed" (DISTR-SEQ) active electrodes, to reduce fatigue (torque-decline) by reducing motor unit discharge rates. The primary objective was to compare fatigue-related outcomes, "potentiation," "variability," and "efficiency" between CONV, AGGR-SEQ, and DISTR-SEQ stimulation of knee extensors in healthy participants. Torque and current were recorded during testing with fatiguing trains using each NMES type under isometric and isokinetic (180°/s) conditions. Compared with CONV stimulation, SEQ techniques reduced fatigue-related outcomes, increased potentiation, did not affect variability, and reduced efficiency. SEQ techniques hold promise for reducing fatigue during NMES-based rehabilitation and exercise; however, optimization is required to improve efficiency. Muscle Nerve 56: 271-281, 2017.

Beta 1-integrin ligation and TLR ligation enhance GM-CSF-induced ALDH1A2 expression in dendritic cells, but differentially regulate their anti-inflammatory properties.

Retinoic acid (RA)–producing CD103+ mature dendritic cells (DCs) in mesenteric lymph nodes (MLNs) play crucial roles in gut immunity. GM-CSF and RA contribute to the expression of the RA-producing enzyme ALDH1A2. However, additional signals appeared to be required for inducing ALDH1A2high mature DCs from immature DCs. We found here that TLR ligands (Ls) and immobilized E-cadherin could provide such signals in FLT3-L–generated bone marrow (BM)–derived DCs after treatment with GM-CSF and the RA receptor agonist Am80. The TLR-L-treated DCs produced proinflammatory cytokines unlike normal ALDH1A2high MLN-DCs, whereas the E-cadherin-treated DCs did not. Immobilized VCAM-1 and semaphorin 7 A exerted effects similar to those of E-cadherin. Soluble anti-integrin β1 antibodies or inhibitors of integrin signaling molecules suppressed the effects of these immobilized proteins, whereas immobilized anti-integrin β1 antibodies enhanced the GM-CSF/Am80-induced ALDH1A2 expression without inducing proinflammatory cytokines. Sequential stimulation of splenic pre-DCs with GM-CSF/Am80 and immobilized E-cadherin or anti-integrin β1 antibody also induced differentiation to mature DCs with high ALDH activity. The E-cadherin-treated BM-DCs induced gut-tropic Foxp3+ T cells and alleviated DSS–induced colitis, whereas the TLR-L-treated DCs aggravated DSS–induced colitis. The results suggest that integrin β1-mediated signals contribute to the differentiation and maturation of RA-producing anti-inflammatory DCs.

Reprogramming of TLR7 signaling enhances antitumor NK and cytotoxic T cell responses

ABSTRACTToll-like receptor (TLR) 7 agonists are effective in topical application for the immunotherapy of skin cancers, but their performance for the systemic treatment of solid tumors is limited by the development of TLR tolerance. In this study, we describe a novel strategy to overcome TLR tolerance and enhance TLR7-dependent antitumor immune responses through reprogramming of TLR signaling pathways. The sensitivity of TLR7 signaling in dendritic cells (DC) was increased by prior stimulation with the dsRNA poly(I:C) that mimics virally induced immune activation. Timing of the stimulations was important, as sequential stimulation with poly(I:C) and the TLR7 agonist R848 interspaced by 24 h induced higher MAPK and NFkB signaling in DC than the simultaneous application of the same ligands. DC activated by sequential poly(I:C)/R848 stimulation efficiently induced Th1 differentiation and primed NK-cell and cytotoxic T-cell responses. We have developed a treatment regimen taking advantage of TLR7 reprogram-ming that cured over 80% of large immunogenic tumors in mice by the action of NK cells and cytotoxic T cells. These results have direct implications for the use of these clinically established ligands in the immunotherapy of cancer.

Cortical responses following simultaneous and sequential retinal neurostimulation with different return configurations.

Researchers continue to develop visual prostheses towards safer and more efficacious systems. However limitations still exist in the number of stimulating channels that can be integrated. Therefore there is a need for spatial and time multiplexing techniques to provide improved performance of the current technology. In particular, bright and high-contrast visual scenes may require simultaneous activation of several electrodes. In this research, a 24-electrode array was suprachoroidally implanted in three normally-sighted cats. Multi-unit activity was recorded from the primary visual cortex. Four stimulation strategies were contrasted to provide activation of seven electrodes arranged hexagonally: simultaneous monopolar, sequential monopolar, sequential bipolar and hexapolar. Both monopolar configurations showed similar cortical activation maps. Hexapolar and sequential bipolar configurations activated a lower number of cortical channels. Overall, the return configuration played a more relevant role in cortical activation than time multiplexing and thus, rapid sequential stimulation may assist in reducing the number of channels required to activate large retinal areas.

A hybrid discrete/finite element modeling study of complex hydraulic fracture development for enhanced geothermal systems (EGS) in granitic basements

The technical and economic success of enhanced geothermal systems (EGS) in granitic basement rocks depends on the ability to effectively develop complex fracture networks by hydraulic stimulation treatments. To study hydraulic fracture growth horizontally, two dimensional discrete element models were set up for the conditions expected in Precambrian basement rocks in Northern Alberta. The simulated fracture networks were then transferred to a finite element reservoir model to investigate the thermal and hydraulic efficiencies of the fracture networks. Natural and engineering factors influencing stimulated fracture network complexity are summarized and their significance is discussed based on the simulation results. The numerical results fit reasonably well to major field observations and led to a proposed reservoir stimulation guideline. This guideline involves drilling wells with a horizontal segment at reservoir depth, performing multiple sequential stimulation treatments alternating between two wells with an offset between the stages in both wells, and a long continuous injection of low viscosity fluid at a high rate without proppants. Favorable drilling targets for the development of complex fracture networks are brittle rocks in complex tectonic settings that are characterized by at least two intersecting natural fracture sets that are favorably oriented to the in-situ stress field, a low difference between the major principal stresses, and a low reservoir permeability. The reservoir simulations show the benefit of developing well connected complex fracture networks compared to single parallel hydraulic fractures.