Parameter substantiation of supra bit jet pump for productive formation opening

The purpose of the study is to develop a supra bit jet pump taking into account the unsteadiness of low-speed drilling for crushing the cuttings injected from the annular space under productive formation opening. The article proposes a device for drill string bottom assembly intended for the initial opening of the productive formation. The device includes a supra bit jet pump and a colmatator. The jet pump creates an additional circulation loop of the drilling fluid above the well bottom, crushes the cuttings injected from the annular space in the mixing chamber and delivers it to the colmatator. An additional circulation loop above the well bottom creates a local drawdown of the formation while maintaining the hydrostatic pressure in the well. Crushing of cuttings in the mixing chamber of the jet pump occurs due to the creation of cross flows in the jet pump. The cross flows are provided due to the angular and eccentric displacement of the working nozzle of the jet pump relative to the mixing chamber. The colmatator creates an impermeable screen on the borehole wall for temporary isolation of the productive formation under initial opening. The conducted study allowed the authors to propose head characteristics of the jet pump taking into account the angular, eccentric displacement of the working nozzle. The head characteristic of the jet pump has been developed for the unsteady operation of the jet pump in the drill string bottom assembly. The head characteristics take into account the roughness of the flow path of the jet pump. Using the head characteristics, the permissible displacements of the working nozzle of the jet pump have been determined. Recommendations for the design of jet pumps for drill string bottom assemblies are proposed.

Hard Backreaming Due to Hole Shrinkage Through Carbonate Reservoir in Offshore Abu Dhabi

The 3,000 ft long lateral holes drilled through water-injected area in the carbonate reservoir in the offshore Abu Dhabi have been forced to implement hard backreaming. The abnormal extra operational time has been taken due to poor performance in the operation to pull out a bottomhole assembly after drilling to the total depth. The study aims to analyze root-causes of the hard backreaming through the carbonate reservoir in the studied field. The speed of tripping-out was analyzed every stand of drill pipe by using time domain data of movement of traveling block. The correlations between the speed of tripping-out and rock characteristics such as porosity and constituent minerals in rocks were investigated. Hole shape was analyzed in the representative intervals of low trip-out speed using 16-sector caliper derived from azimuthal density logging. Stress concentration around the borehole wall was also analyzed using geomechanical model. The investigation revealed that hole shrinkage due to plastic deformation of the borehole wall was the most possible root-cause of the hard backreaming in the carbonate reservoir. Namely, BHA had to ream up deformed borehole wall in tripping-out. From the viewpoint of rock characteristics, the speed of tripping-out was found to be lower in the specific geologic layers with higher content of dolomite. This is because dolomite rocks cause larger resistance in reaming it in tripping-out since the strength of dolomite rocks is larger than that of limestone. Based on our findings, use of reamers on bit is found to be the better solution to improve the tripping-out performance in the problematic geologic layers instead of conventional operational attempts such as spotting of acid and use of high viscous fluids in hole cleaning. In addition, optimization of the design and position of reamers and stabilizers is essential to succeed in the future 10,000 ft long extended-reach wells in the studied oil field.

Geomechanical Modeling Based on the First Success of Micro-Frac Tests in the Nahr Umr Formation in Offshore Abu Dhabi

In the studied oil field in Offshore Abu Dhabi, the intermediate hole section has suffered from borehole instability and lost circulation in the higher inclination holes. Borehole instability occurs in the Nahr Umr formation. Lost circulation occurs in the Salabikh formation. This study aims to develop geomechanical model and to analyze mud weight (MW) for successful drilling through the two problematic formations in the studied oil field. In the Salabikh formation, spatial distribution of lost circulation pressure in hundreds of wells in the whole field was analyzed. The fracture closure pressure was also evaluated based on the extended leak-off test and fracture interpretation by image logging. In the Nahr Umr formation, Micro-Frac tests in a 6" hole were implemented to evaluate the minimum in-situ stress. This was the first direct measurement of the in-situ stress in the shale. The magnitude of SHMAX was back-analyzed based on the hole geometry using interpretation of six-arm caliper and analytical solution in the two key locations. This study clarified that severe lost circulation in the crest area was likely to occur due to reactivation of the pre-existing fractures in the Salabikh formation. The lost circulation pressure was found to be approximately 1.4 SG. The study also revealed that the in-situ stress regime in the Nahr Umr formation varied from the crest to flank areas. The crest and flank areas are reverse and nearly normal faulting stress regimes, respectively. Its transition area is strike-slip faulting stress regime. The regional difference in in-situ stress regime depends on the extent of mechanical anisotropy of the shale and the magnitude of tectonic strains. By integrating the results, with respect to the borehole stability analysis in the Nahr Umr formation, instead of a conventional lower hemisphere representation of the required MW based on failure width at borehole wall, the study analyzed the geometry of the failure area around the borehole wall under the allowable range of MW constrained by the lost circulation pressure in the Salabikh formation. As a result, the borehole failure cannot be avoided in any hole inclination in the Nahr Umr formation under the allowable range of MW to prevent severe lost circulation in the Salabikh formation. Therefore, appropriate practice to transport cavings is one of the key elements for safe drilling in higher hole inclination across the intermediate hole section in the studied oil field.

An Analytical Model for Transient Heat Transfer with a Time-Dependent Boundary in Solar- and Waste-Heat-Assisted Geothermal Borehole Systems: From Single to Multiple Boreholes

With the increasing engineering applications of geothermal borehole heat exchangers (BHEs), accurate and reliable mathematical models can help advance their thermal design and operations. In this study, an analytical model with a time-dependent heat flux boundary condition on the borehole wall is developed, capable of predicting the thermal performance of single, double, and multiple closed-loop BHEs, with an emphasis on solar- and waste-heat-assisted geothermal borehole systems (S-GBS and W-GBS) for energy storage. This analytical framework begins with a one-dimensional transient heat conduction problem subjected to a time-dependent heat flux for a single borehole. The single borehole scenario is then extended to multiple boreholes by exploiting lines of symmetry (or thermal superposition). A final expression of the temperature distribution along the center line is attained for single, double, and multiple boreholes, which is verified with a two-dimensional finite-element numerical model (less than 0.7% mean absolute deviation) and uses much lesser computational power and time. The analytical solution is also validated against a field-scale experiment from the literature regarding the borehole and ground temperatures at different time frames, with an absolute error below 6.3%. Further, the thermal performance of S-GBS and W-GBS is compared for a 3-by-3 borehole configuration using the analytical model to ensure its versatility in thermal energy storage. It is concluded that our proposed analytical framework can rapidly evaluate closed-loop geothermal BHEs, regardless of the numbers of boreholes and the type of the heat flux on the borehole wall.

Ultrasonic Slowness Images for Geological Interpretation: Investigating Field Test Examples of a New LWD Technology

Borehole images have long been used for various applications related to geological interpretation, geomechanics and well-placement objectives. Based on the physical measurement used, the resolution and detection limit of the image logs vary considerably thereby limiting or enhancing the applications envelope against specific objectives. A new logging-while-drilling (LWD) technology of pitch-catch (PC) ultrasonic is under development, that can also provide borehole images of compressional and shear slowness in anisotropic and / or heterogenous formations both in water and oil-based-mud. Geological application of these images (which have inch-scale resolution) is being assessed and investigated with respect to the already established ultrasonic pulse-echo (PE) LWD images. While PE images show details of the borehole wall and are thus subject to the effects of drilling practices, the PC images reflect the rock mechanical properties of the formation at the depth of investigation of the measurement, in the same fashion as density images. Two case-studies are presented to assess the quality and application of these images; with one vertical and one horizontal well. The PC images are shown to provide a lot of meaningful information in addition to PE images, though the PC images lose out on detailed information related to texture and subtle structural features. In fact, the gross geological features of bedding and some sedimentation sequences have been imaged with confidence, with manifestation of the features available across the entire azimuthal coverage with the new-technology images and confident dips with sinusoidal fits are picked for gross geological variations. The horizontal well PC images have picked up higher confidence low-angled features than the vertical well PC images due to the longer intersection with borehole wall. The slowness images are found to provide confident bed-boundaries, also some fracture-traces are visible only on the slowness images. However, major vertical fractures and drilling-induced fractures (DIF) are more prominent on the PE amplitude images. The results show that slowness images offer the capability to understand gross structural dip and facies variability at a better resolution than conventional LWD images of similar resolution, and with the benefit of not needing to utilize a chemical source in the drilling bottom-hole assembly (BHA) for density images. Although the slowness measurements azimuthally have been developed to investigate near wellbore geomechanical concerns, such as stress, heterogeneity, and anisotropy, it is clear that the resultant images also have geological applications that is demonstrated by the datasets shown. These applications mean that the images could offer a more sustainable geosteering capability if delivered in real-time.

ENHANCING THE RESERVOIR PERFORMANCE BY ACCESSING THE RESERVOIR SWEET SPOTS GUIDED BY FAR-FIELD SONIC IMAGING: AN INTEGRATED CASE STUDY FROM THE NORWEGIAN NORTH SEA

The gas present in the Valhall overburden crest area interferes with the seismic data and obscures the fault detection (minor faults). Spatially resolving fractures and fracture network is essential for subsurface understanding and future well placement in this field, and it is a critical input to the dynamic reservoir model. Additionally, mapping the fracture network in poor permeable reservoir formation beyond the wellbore is crucial to identify completion intervals to maximize productivity/injectivity, and hence field value. The well 2/8-F-18 A was drilled on the crest of the Valhall field as a pilot water injector in Lower Hod formation, where core and data analysis formed the foundation for a future potential 11 well development. The well is placed in the southern section of the Valhall crest, and no major faults or strong amplitude features were mapped out in the overburden via surface seismic before drilling. In this case study, an integrated workflow is proposed and tested within the reservoir formation to identify “sweet” (permeable and fractured) zones beyond the wellbore. This is achieved using borehole acoustic data combined with image and ultrasonic imaging to characterize fracture networks beyond the borehole wall. The sonic imaging workflow identifies reflection events from fractures and faults and provides the true dip, azimuth, and location in 3-dimensions. This data is complemented by nuclear magnetic resonance (NMR), dielectric and spectroscopy data to understand reservoir petrophysics. NMR-derived permeability has also been evaluated for identifying high permeable zone in this formation, which primarily focuses on intergranular permeability of the formation a few inches away from the borehole wall. Reservoir textural heterogeneity and fractures beyond the wellbore wall make this method difficult to estimate or enhance the effective permeability estimate. The baseline assumption for the NMR permeability estimation is also not valid in Hod formation; the Timur and SDR equation needs significant change to match core permeability. Hence, the primary aim is to identify a fracture network that will help support water injection and maximize hydrocarbons production through them. The goal is to establish a workflow from the learnings of this study, performed on the pilot well, validate its findings with the near-field data (core, imaging, and ultrasonic), and optimize it if needed (described in the methodology section). The developed workflow is then intended to be used to optimize the placement of future wells. The results achieved from the integrated workflow identified a key fault and mapped it approximately 23 meters away on each side of the borehole. It also captures acoustic anomalies (high amplitudes), validated based on near-field data, resulting from a fracture network potentially filled with hydrocarbons. The final results show the sub-seismic resolution of the fracture and fault network not visible on surface seismic due to the gas cloud above the reservoir and frequency effect on the surface seismic when compared to borehole sonic data. Evidently enhancing the blurred surface image, which helps enhance the structural and dynamic model of the reservoir.