The use of Distributed Acoustic Sensing for Strain Fronts (DAS-SF) is gaining popularity as one of the tools to help characterize the geometries of hydraulic fracs and to assess the far-field efficiencies of stimulation operations in Unconventional Reservoirs. These strain fronts are caused by deformation of the rock during hydraulic fracture stimulation (HFS) which produces a characteristic strain signature measurable by interrogating a glass fiber in wells instrumented with a fiber optic (FO) cable cemented behind casing. This DAS application was first developed by Shell and OptaSense from datasets acquired in the Groundbirch Montney in Canada. In this paper we show examples of DAS-SF in wells stimulated for a variety of completion systems: plug-and-perforating (PnP), open hole packer sleeves (OHPS), as well as, data from a well completed via both ball-activated cemented single point entry sleeves (Ba-cSPES) and coil-tubing activated cemented single point entry sleeves (CTa-cSPES). By measuring the strain fronts during stimulation from nearby offset wells, it was observed that most stimulated stages produced far-field strain gradient responses in the monitor well. When mapped in space, the strain responses were found to agree with and confirm the dominant planar fracture geometry proposed for the Montney, with hydraulic fractures propagating in a direction perpendicular to the minimum stress. However; several unexpected and inconsistent off-azimuth events were also observed during the offset well stimulations in which the strain fronts were detected at locations already stimulated by previous stages. Through further integration and the analysis of multiple data sources, it was discovered that these strain events corresponded with stage isolation defects in the stimulated well, leading to "re-stimulation" of prior fracs and inefficient resource development. The strain front monitoring in the Montney has provided greater confidence in the planar fracture geometry hypothesis for this formation. The high resolution frac geometry information provided by DAS-SF away from the wellbore in the far-field has also enabled us to improve stage offsetting and well azimuth strategies. In addition, identifying the re-stimulation and loss of resource access that occurs with poor stage isolation also shows opportunities for improvement in future completion programs. This in turn, should allow us to optimize operational decisions to more effectively access the intended resource volumes. These datasets show how monitoring high-resolution deformation via FO combined with the integration of other data can provide high confidence insights about stimulation efficiency, frac geometry and well construction defects not available via other means.
The traditional approach to control education in Universities has been to enhance student learning with hardware style experiments. The associated experiments are always constrained by the fact that hardware must be provided. Thus typical experiments use tanks of water, servo motors, inverted pendula etc. These experiments are good in so far as they go. However, quoting a former student, "It is a bit like learning to fly a Jumbo Jet. One has the choice to learn on real hardware (say an ultralight aircraft) or on a simulator of the real aircraft under real flight scenarios". This paper explores this issue for control education and presents feedback from students comparing traditional hardware experiments with simulated experiments based around real world control system designs.
It has been widely demonstrated that frac stimulation efficiency and more importantly production, varies significantly between perforation clusters as well as between sleeve entries. Recent trends indicate that many operators are simultaneously increasing the number of perforation clusters or entries while decreasing frac-to-frac spacing. This is done with the expectation that it will lead to more productive wells overall. The purpose of this paper is to investigate some of the aspects that may limit this approach. There are an increasing number of frac diagnostic tools which allow us to get a better understanding of frac placement and production. Unfortunately, there are only few diagnostic tools available today to characterize the near wellbore region (NWR). Fiber Optics (FO) and other downhole measurements can play an important role in providing information about the NWR. In this paper, we share data and examples from wells where the combination of data from Distributed Acoustics Sensing (DAS), Distributed Temperature Sensing (DTS) and downhole gauges is helping us gain insights about this poorly understood region of our unconventional reservoirs. This paper combines DAS, DTS and downhole pressure gauge data to demonstrate the existence of significant near wellbore complexity, both during stimulation and production. We frequently observe changes in DAS signal and pressure during the stimulation of horizontal wells completed via both "Plug and Perf" (PnP) and Cemented Single Point Entry (CSPE) systems. These changes support the existence of significant near-wellbore tortuosity. Furthermore, we show that pressure data from downhole gauges can differ significantly from surface pressure data extrapolated downhole. This can impact the interpretation of Step-Down-Tests, other analytical techniques relying on the surface pressure alone and affecting the calibration of frac models aimed at understanding the NWR. In wells instrumented with a FO cable behind casing, it is possible to use the DTS data during warmback, following stimulation injection to gain insights about frac geometry in the NWR. Such data provides information about the hydraulic frac dimensions created by the stimulation process in both vertical and horizontal wells. During warmback it is easy to distinguish intervals containing hydraulic fractures near the wellbore where the temperature recovery is lagging compared to the unstimulated portions of the well. FO instrumented horizontal wells allow for estimation of the dimensions of the "Frac-Zone" along the wellbore in the NWR where a combination of hydraulically induced longiditunal and vertical transverse fracs exist. Thermal modeling is also presented for selected stages that further support the qualitative interpretation of the DTS. The diagnostics presented help quantify the dimensions of longitudinal and transverse components in horizontal wellbores in the NWR. This paper also highlights the risk of putting perforation clusters or sleeve entries too close to one another. It is clear that the NWR is poorly understood and more information is needed. Understanding the processes that govern the NWR are essential, after all, this is the region where the well and the reservoir interact.
Comprehensive and integrated diagnostics associated with the NETL-GTI Hydraulic Fracture Test Site 2 (HFTS2) in the Permian Delaware Basin enabled a unique opportunity to evaluate stimulation distribution effectiveness (SDE) in the wellbore, near-well and far-field regions (WBR, NWR, FFR) for upper Wolfcamp completions. This paper will summarize the experimental design, execution and analysis for HFTS2 well completions. Results and observations are translated into applications and opportunities for Wolfcamp completions and resource development optimization. A Completions Sub-Committee was created to allow for broad industry input to design the scope of trials to be considered for execution in the HFTS2 wells. Primary objectives were to evaluate Stimulation Distribution Effectiveness (SDE) and test eXtended Stage Lengths (XLS) for stimulation value opportunities. Trials included: 1) HFTS2 operator base-plan completions; 2) more Aggressive Limited Entry (ALE) practices; 3) tapered number of perforations per cluster; and 4) XLS up to ∼ 330’ stage lengths, with ALE practices. Diagnostics included multiple wells with Optic Fiber and Step-Down Tests to evaluate stimulation domain characteristics in the WBR, NWR and FFR regions. The combination of multiple wells with permanent optic fiber installed, and controlled sequencing the stimulation treatment placement operations, enabled good understanding of the frac domains created in the WBR, NWR and FFR. Evaluation of the base-design completions demonstrated good SDE for most applications, from the optic fiber distributed sensing. EXtreme-Limited-Entry (XLE) applications were determined to not be necessary for SDE in the Permian Wolfcamp. Tapered perforation designs were not necessary for good SDE. Risks to SDE were clearly observed with injectivity loss due to mechanical problems with the pumping operations. Risks to SDE were also observed when stage isolation was not effective. Extended Stage Lengths (XLS), up to ∼ 330’ stage lengths with 10 perforation clusters, were generally effective with ALE practices. Acquiring and maintaining injection rate was critical for the higher cluster count stages. The FFR stimulation domain dimensions were generally consistent with the WBR dimension distributions. Significant non-uniformity was observed in the FFR at a cluster dimension resolution. There was good correlation with domain dimensions in the FFR when there was lack of stage isolation the treatment well. Staggered landing depths of the wells, and an instrumented vertical monitor well, enabled assessment of vertical fracture geometry characteristics. Application of integrated diagnostics for completion and stimulation design evaluation enabled assessment of multiple designs in just a few wells. Optic Fiber Distributed Acoustic, Temperature and Cross-Well Strain Sensing (DAS, DTS & DSS), in combination with selective Step-Down Tests, enabled evaluation of stimulation domain characteristics in the WBR, NWR and FFR regions. The results and observations enabled greater confidence for timely completions, stimulations and resource development applications for the Permian Delaware Basin.
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