Uphole Data Research Articles

An airborne micro-TEM and physics deep learning solution to near-surface corrections in sand-covered areas

Sand-covered areas, such as desert environments, pose challenges to seismic imaging for resource exploration and monitoring. Aeolian sand dunes provide extreme variations of elastic parameters causing, among other effects, nonlinear velocity gradients proportional to compaction. Description and correction for sand velocities is typically performed through empirically derived sand curve functions. Shallow drilling with uphole traveltime measurements represents another established technique for characterizing the thickness of the sand layer. This defines a basal depositional surface across the investigated area to apply the sand correction for seismic processing operations. The accurate definition of the unconformable base of sand is crucial for the application of robust near-surface corrections. An airborne transient electromagnetic solution is proposed, where the acquisition instrumentation, acquisition parameters, and inversion strategies are tuned to the ultra-shallow depth of the target (2–20 m). The data inversion component makes extensive use of a recently introduced physics-driven DL inversion scheme. It enables boosted depth resolution in the resistivity model using all of the available data while achieving an acceptable data misfit to honor the propagation of the electromagnetic radiation. The imaged resistivity boundary provides an excellent match to the existing uphole data and indicates the presence of long-wavelength mismatch from the previously interpreted base of sand.

Enhanced surface seismic data processing using simultaneous acquired DAS-VSP data

Distributed Acoustic Sensor (DAS) is increasingly recognized as a viable alternative to downhole geophone arrays for the acquisition of borehole seismic (VSP – Vertical Seismic Profile) or DAS-VSP data. When acquiring surface seismic data in the field, simultaneously acquired DAS-VSP data in an available well or wells can provide full well VSP imaging and significantly enhance surface seismic data processing. Borehole seismic data can provide higher resolution structural images around the borehole as the receivers are located inside a borehole and near the reservoir. In addition, the formation velocity, deconvolution operator, absorption, attenuation (Q value), anisotropy parameters and other reservoir formation information in a certain range around the wellbore can be obtained directly from the VSP data. These can be used to significantly enhance the surface 3D seismic data processing, such as velocity model calibration and modification, static correction, deconvolution, demultiple processing, high frequency restoration, anisotropic migration, and Q-compensation or Q-migration. In this project, Q-migration was conducted with the enhanced Q-field data volume obtained from the joint inversion of both the near surface 3D Q-field data volume from uphole data and the mid-deep layer Q-field data volume from VSP data. The Q-migration results show much sharper and more focused faults and fracture zones. A new 3D formation top structural model was built with much finer details based on the Q-migration data, and three new borehole locations were recommended to the client. The final drilling results on these three new boreholes are very successful with high volume of commercial oil flow.

English

The near-surface anisotropic properties within the South-Western Niger Delta were investigated to determine their impacts on geotechnical constructions. Uphole data analyses reveal a weathered layer that is thickest in the NW, E and SE with an average of 4.2 m and thinnest in the North-Central with a value of 3.8 m; a sub-weathered layer that is thickest in the NW and SW with an average of 32.1 m and thinnest in the SE with a value of 19.0 m and an underlying consolidated layer with great thickness. Vp/Vs values of 1.16265, 1.16279 and 1.16278 were computed for the layers, respectively. Similarly, Poisson’s ratio of -0.9214, -0.9201 and -0.9202 were obtained for the layers respectively. Shear modulus has an average of 4.01 × 108 Nm-2 in the weathered layer, 26.0 × 108 Nm-2 in the sub-weathered layer, and 49.98 × 108 Nm-2 in the consolidated layer. Young’s modulus has an average of 0.63 × 108, 4.11 × 108 and 7.98 × 108 Nm-2 in the layers, respectively. Bulk modulus has an average of 0.74 × 108 Nm-2, 4.62 × 108 Nm-2 and 0.94 × 108 Nm-2 in the layers, respectively. The ultimate bearing capacity, qf has an average of 804.9, 2241.0 and 3349.8 kNm-2 in the layers, respectively. The allowable bearing pressure, qa of the soil has an average of 201.2, 560.2 and 837.4 kNm-2 in the layers, respectively. Core samples collected to a depth of 66 m from 12 wells revealed an admixture of sand, sandy-clay, clayey-sand and dominant clay units. These quantitative results correlate with the standard geotechnical values for clay. The large deposit of about 30 m of clay delineated in this research may act as expansive soils which portends danger for foundations of buildings located in this area and must be avoided. Key words: Anisotropy, uphole, near-surface, clay, geotechnical, Niger Delta.

Analysis of Time-Depth Data in Sand Dunes from the Empty Quarter Desert of Southeastern Saudi Arabia

Theoretical and field studies show that P-wave velocity increases continuously with depth in unconsolidated sands. Many analytical and empirical models have been proposed to account for this phenomenon in near-surface layers. This study presents five new uphole data sets acquired in sand dunes located in the Empty Quarter (Rub’ Al-Khali) Desert in southeastern Saudi Arabia. Furthermore, the study compares the following velocity-depth functions: constant-velocity, Al-Shuhail, and Liner by fitting them to these data sets in addition to the sand curve data set. The constant-velocity function estimates an average P-wave velocity equal to 641 m/s in the dunes with an average sum of squared errors \(\hbox {(SSE)} = 0.00101\). Liner’s velocity function estimates an average surface P-wave velocity = 450 m/s and an average porosity (\(\phi ) = 38.33\%\) in the dunes with an average \(\hbox {SSE} = 0.000133\). Al-Shuhail’s velocity function estimates an average surface P-wave velocity \( = 25.78\,\hbox {m/s}\) and an average velocity-depth gradient \( = 662\,\hbox { m}^{-1/3}\) in the dunes with an average \(\hbox {SSE} = 0.0000877\).

Geostatistical integration of near‐surface geophysical data

ABSTRACTAccurate statics calculation and near‐surface related noise removal require a detailed knowledge of the near‐surface velocity field. Conventional seismic surveys currently are not designed to provide this information, and 3D high‐resolution reflection/refraction acquisition is not feasible for large survey areas. Satellite images and vibrator plate attributes are dense low‐cost data, which can be used in spatially extrapolating velocities from sparse uphole data by geostatistics. We tested this approach in two different areas of Saudi Arabia and found that the optimal recipe depends on the local geology.

Ground viscosity and stiffness measurements for near surface seismic velocity

ABSTRACTIn land surveys, the weathering layer can often distort the seismic signal due to it passing through rapid velocity and density changes, dispersion, scattering and inelastic absorption. In a simple spring‐dashpot model for the earth response, an equivalent medium groups these complex phenomena into two parameters only; these are called ground viscosity and ground stiffness. The most recent controllers for vibrators can estimate both parameters. To validate these measurements, Saudi Aramco conducted an experiment measuring ground viscosity and stiffness from two different vibrator control systems over an area of varying terrain conditions, including unconsolidated sand and limestone outcrop. The two systems measured different values, but detected similar trends that correlated well with weathering conditions and surface geology, e.g. lower viscosity values on the outcrop than on the sand.The ratio of ground viscosity to ground stiffness can approximate the shallow S‐wave velocity, which we converted into P‐wave velocity through calibration with sparse uphole data. Static corrections incorporating this velocity information somewhat improved the focusing of seismic time sections. This new approach does not require additional acquisition efforts, and can model shallow complex formations in arid areas where classical static methods often fail.

Advances in Openhole Well Logging

Distinguished Author Series Summary Openhole well logging comprises a broad spectrum of measurements that includes mud, measurement-while-drilling (MWD), and conventional wireline logs. These measurements are the primary source of formation evaluation data used in applications ranging from individual drilling-well appraisals to comprehensive reservoir description studies. Openhole well-logging technology continues to evolve to address the needs for improved accuracy in measuring reservoir properties. Numerous technology advances have recently occurred. This paper presents several examples of new developments and applications to illustrate the current state of technology and to offer insight into future developmenttrends. Introduction Significant developments have recently been made in openhole well-logging technology, despite a major downsizing in the industry. New measurement tools and software systems have been introduced in an effort to provide accurate, cost-effective methods for addressing a variety of formation evaluation challenges, which include thin-bed evaluation, low-resistivity pay analysis, productivity/permeability prediction, horizontal well evaluation, fractured reservoir analysis, and reservoir geology characterization. Advances have been made in a variety of logging technologies. Enhancements in log data acquisition include higher data sampling and transmission rates and expanded computer capabilities to process log data and to control logging operations. The latest borehole imaging tools offer improved image quality from both microresistivity and acoustic imaging logs, which has led to growing applications. New multielectrode induction and laterologs provide enhanced thin-bed resolution and new formation measurements. Source, detector, and processing enhancements offer improved porosity and elemental analysis accuracy from nuclear logging tools. New acoustic logs provide multipole source and receiver arrays to measure compressional, shear, and Stoneley wave data in both hard and soft formations. In addition, new nuclear magnetic resonance logs offer greatly improved data quality for determining porosity, irreducible water saturation, and other formation and fluid properties. Wireline formation tester developments provide new downhole measurements, pump-out capabilities, and a modular design that allows multiple configurations. Advances in MWD logging include new resistivity measurements, improved porosity/lithology logs, and new applications. Finally, developments in log data processing and interpretation provide faster processing, more information from the log measurements, and more effective integration of data from multiple sources, such as drilling, mud logging, core analysis, geology, testing, and production. Data Acquisition The growing trend of increased numbers of downhole sensors and greater measurement sampling frequency for improved resolution has put new demands on well-log data acquisition systems. These systems comprise the hardware and software for sending data uphole from the logging tool; for receiving, processing, verifying, presenting, and storing data at the surface; and for electronically transmitting data from wellsite to office locations. Developments in downhole signal processing and data compression techniques are dramatically increasing the uphole data transmission rates to meet the requirements of new measurement tools. New wireline systems provide rates exceeding 500 kbit/sec, and future systems offering several megabits per second are expected in the near term. MWD transmission rates, based on mud-pulsetelemetry, have also shown improvement, but they remain a limiting factor for real-time data acquisition in some cases. MWD data transmission rates have recently increased from 3 to 12 bits/sec, and further improvements are foreseen. The capabilities of computer surface equipment to process log data and to control logging operations have also increased significantly. This has resulted in more efficient logging operations, improved quality control of log data, enhanced wellsite log analysis products, and improved methods for storing and transmitting data. These improvements have been made possible by using high-speed, multitasking workstations in wellsite logging units. Borehole Imaging Significant improvements have recently been made in the quality of borehole imaging measurements, which have led to growing applications of this technology. These measurements are commonly used for thin-bed evaluation, fracture identification and analysis, structural and stratigraphic dipanalysis, sedimentary facies analysis, and textural analysis. Technology developments have led to (1) better assessment of reserves in thinly bedded reservoirs; (2) improved definition of fracture density, aperture, and orientation for producibility estimation and perforation placement; and (3)greater geological information, such as definition of structural dip, paleocurrent direction, rock facies, flow barriers, and porosity types. Ref. 4gives a good overview of borehole imaging technology and its applications and contains a collection of technical papers describing the development of borehole imaging from its beginning until 1989. The primary techniques for acquiring borehole image data use electrical (microresistivity) or acoustic measurements. Microresistivity Imaging. Microresistivity imaging tools produce an image of the borehole wall by mapping its resistivity with an array of small, pad-mounted, button electrodes. P. 693^

Application of uphole data from petroleum seismic surveys to grounwater investigations, Abu Dhabi (United Arab Emirates) : Woodward, D; Menges, C M GeoexplorationV27, N1/2, Feb 1991, P193–212 (paper to the International Symposium on Application of Geophysics to Water Prospecting in Arid and Semi-Arid Areas, Lausanne, 22–24 August 1990)

Application of uphole data from petroleum seismic surveys to grounwater investigations, Abu Dhabi (United Arab Emirates) : Woodward, D; Menges, C M GeoexplorationV27, N1/2, Feb 1991, P193–212 (paper to the International Symposium on Application of Geophysics to Water Prospecting in Arid and Semi-Arid Areas, Lausanne, 22–24 August 1990)

Application of uphole data from petroleum seismic surveys to groundwater investigations, Abu Dhabi (United Arab Emirates)

Velocity data from uphole surveys were used to map the water table and the contact at the base dune sand/top alluvium as part of a joint National Drilling Company-United States Geological Survey Ground Water Research Project in the Emirate of Abu Dhabi. During 1981–1983, a reconnaissance seismic survey was conducted for petroleum exploration in the eastern region of Abu Dhabi. Approximately 2800 kilometers of seismic data, consisting of 92 lines, were acquired in the 2500 km 2 concession area near Al Ain. Uphole surveys were conducted about 2 km apart along each seismic line, and were used to calculate weathering corrections required to further process in the seismic data. Approximately 1300 uphole surveys were completed in the concession area between March 1981 and June 1983. Reinterpretation of the velocity profiles derived from the uphole surveys provided data for determining the following subsurface layers, listed in descending order: (1) a surficial, unconsolidated weathering layer with a velocity from 300 to 450 m/s; (2) surficial dune sand, from 750 to 900 m/s; (3) unsaturated, unconsolidated alluvium, from 1000 to 1300 m/s; and (4) saturated, unconsolidated alluvium, from 1900 to 2200 m/s. Two interfaces—the water table and the base dune sand/top alluvium — were identified and mapped from boundaries between these velocity layers. Although the regional water table can fluctuate naturally as much as 3 m per year in this area and the water-table determinations from the uphole data span a 27-month period, an extremely consistent and interpretable water-table map was derived from the uphole data throughout the entire concession area. In the northern part of the area, unconfined groundwater moves northward and northwestward toward the Arabian Gulf; and in the central and southern parts of the area, groundwater moves westward away from the Oman Mountains. In the extreme southern area east of Jabal Hafit, groundwater moves southward into Oman. The map of the base dune sand/top alluvium suggests a buried paleodrainage network trending westward to southwestward away from the Oman Mountains. These paleodrainages, now buried by dune sand, probably contain alluvial fill and are logical targets for groundwater exploration.

Determining Subsurface Stratigraphic Position by Fluid-Entry Analysis During Air Drilling

The most common methods of determining subsurface stratigraphic position while drilling with mud are difficult or impossible to use in air-drilling operations because drill cuttings may be powdery, and the extremely fast drilling rate remains unchanged as different formations are penetrated. Moreover, air-drilled holes are commonly completed in 4 to 10 days and logs are run only at total depth. These factors demand a wellsite specialist and a rigorous attempt to monitor tops, if formation depths cannot be accurately predicted. A different type of subsurface stratigraphic surveillance, applicable only to air-drilling, has proved successful. Rather than attempting to monitor proved successful. Rather than attempting to monitor the rock characteristics of marker zones, the method monitors their fluid content. Since air compression is stopped and the well is opened to the atmosphere while drillpipe connections are made, drilling with air is like conducting a series of 30-ft formation tests. This fact, together with the fact that bottom-hole air pressures rarely reach 500 psi and bottoms-up times pressures rarely reach 500 psi and bottoms-up times last only a few minutes, makes it possible to observe the type and depth of fluids feeding in from penetrated formations. By comparing fluid-entry depths with logs, the fluid content of specific formations in an area can be determined. It follows that this information may provide a means of fixing the geologic position of a well by monitoring fluids that enter the wellbore during drilling. Thus, by comparing, the nature and distances between fluids entering the wellbore with known fluid-entry characteristics for the area, the subsurface stratigraphic position can be deduced. This technique is termed fluid-entry analysis. The method has been applied by Continental Oil Co. on the thickly faulted Douglas Creek Arch in northwestern Colorado and has provided sufficiently accurate subsurface control to allow substantial savings in spite of otherwise unpredictable formation depths. In air drilling to the Mancos "B" gas reservoir, it became evident that specific up-hole formations contributed fluids to the wellbore in a fairly predictable manner. From this, a system was devised to predictable manner. From this, a system was devised to monitor subsurface position during drilling. In planning a well, tops are predicted from the structure contour map and surface elevation. The well is drilled with air or mist, and depths at which water and gas occur, as well as increases in water and gas, are noted. Fluid-entry data are compared with known fluid-entry characteristics and the original predictions are revised as information is received. predictions are revised as information is received. After the adjusted total depth is reached and a flow test is taken, the well is logged. The only data immediately required are formation productivity and tops, so specially trained personnel are not required on location. The company drilling foreman reports this information and a completion decision is made. Following are the results of using this procedure during the drilling of 40 wells: By not drilling 2593 ft of useless hole in 9 wells that were structurally higher than predicted, $11,000 was saved. Ten wells were drilled deeper than originally planned and were stopped at the correct depth planned and were stopped at the correct depth without interrupting drilling, saving approximately $21,000. Only one log was required and it was not necessary to re-enter and deepen any well after logging. Specially trained personnel were required on location for only 17 man-days as compared with an estimated 100 man-days if conventional surveillance methods had been attempted. Using the method, casing, was not set in any well that was later found noncommercial. Gas feed-in from the Mancos "B" alone might be sufficient to bottom the well properly however, should it be necessary to mud up before reaching the Mancos "B" because of water flow, or should the well not produce gas or flow less than expected at the predicted Mancos "B" depth, the lack of up-hole predicted Mancos "B" depth, the lack of up-hole fluid-entry data would dictate that drilling be halted and that a log be run to determine where the well is geologically. This could require a geologist or engineer at the wellsite and additional expense if it were necessary to re-enter the well and deepen after logging, or if the well had already been drilled too deep. Fluid-entry analysis can be applied wherever two or more marker formations contribute fluids consistently over a large enough area to allow application of the method to a number of wells. The drilling fluid must be air or air mist and fluid feed-in should be of low enough pressure and volume to prevent having to mud up, at least until enough interval has been drilled to determine up-hole stratigraphy. P. 1222