Borehole Gravity Data Research Articles

Three‐axis borehole gravity monitoring for CO2 storage using machine learning coupled to fluid flow simulator

AbstractThe field of geophysics faces the daunting task of monitoring complex reservoir dynamics and imaging carbon dioxide storage up to several decades into the future. This presents numerous challenges, including sensitivity to parameter changes, resolution of obtained results and the cost of long‐term deployment. To effectively store CO2 subsurface, it is necessary to monitor and account for the injected CO2. The gravity method provides several advantages for CO2 monitoring, as changes in fluid saturation correspond directly and uniquely to observed density changes. Three‐axis borehole gravity has demonstrated significant promise as a next‐generation tool for reliably monitoring reservoir dynamics across a range of depths and sizes. However, the gravity inverse problem is highly ill‐posed, necessitating regularization that incorporates prior knowledge. To address this issue, we propose using a feed‐forward neural network, a machine learning method, to invert time‐lapse three‐axis borehole gravity data and monitor CO2 movement within a reservoir. By training the neural network on models that analyse changes in density and corresponding gravity responses resulting from perturbations made to the reservoir model, we can create scenarios that train the algorithm to identify unexpected CO2 migration in addition to the normal movement of CO2. Our method is demonstrated using reservoir models for the Johansen formation in offshore Norway. We convert reservoir saturation models into density changes and generate their corresponding three‐axis gravity data in a set of boreholes. Our results show that the developed machine learning inversion algorithm has high reliability and resolution for imaging density change associated with CO2 plumes, as demonstrated in the Johansen reservoir models utilized by the simulator. We also investigate machine learning inversion using regularization parameters and show that it is robust, with a strong tolerance for higher levels of noise. Our study demonstrates that the developed machine learning algorithm is a powerful tool for inverting three‐axis borehole gravity data and monitoring the migration and long‐term storage of injected CO2.

3D joint inversion of surface and borehole gravity data using zeroth-order minimum entropy regularization

3D joint inversion of surface and borehole gravity data using zeroth-order minimum entropy regularization

Annotation of Using Borehole Time-Lapse Gravity by Genetic Algorithm Inversion for Subsurface Modeling

We present the annotation to a genetic algorithm (GA) method for an inverse synthetic subsurface density model using surface and borehole time-lapse gravity data. The objective of the inversion is to find the boundaries of the object area and background, where one bit of the chromosome represents the densities. The model that was used in this paper was a simple homogeneous body anomaly and a simplified real water mass injection model in order to argue that the code is suitable for field modeling. We show the influences of the existence of borehole gravity data and location towards the inversion, where the result indicates that an additional good borehole location could increase the success rate up to 13.33% compared to without gravity borehole data for the simple model and up to 4.39% for the field model. The inversion produced the best results when the borehole positions were placed in a state of symmetry towards the body object's mass.

3D joint inversion of gravity-gradient and borehole gravity data

Borehole gravity is increasingly used in mineral exploration due to the advent of slim-hole gravimeters. Given the full-tensor gradiometry data available nowadays, joint inversion of surface and borehole data is a logical next step. Here, we base our inversions on cokriging, which is a geostatistical method of estimation where the error variance is minimised by applying cross-correlation between several variables. In this study, the density estimates are derived using gravity-gradient data, borehole gravity and known densities along the borehole as a secondary variable and the density as the primary variable. Cokriging is non-iterative and therefore is computationally efficient. In addition, cokriging inversion provides estimates of the error variance for each model, which allows direct assessment of the inverse model. Examples are shown involving data from a single borehole, from multiple boreholes, and combinations of borehole gravity and gravity-gradient data. The results clearly show that the depth resolution of gravity-gradient inversion can be improved significantly by including borehole data in addition to gravity-gradient data. However, the resolution of borehole data falls off rapidly as the distance between the borehole and the feature of interest increases. In the case where the borehole is far away from the target of interest, the inverted result can be improved by incorporating gravity-gradient data, especially all five independent components for inversion.

Evaluation of formation pore pressure behind the casing using borehole gravity data

Reliable estimates of the fluid pressure in the pore space of rocks are critical for different aspects of petroleum exploration and production including injection operations and scenarios of water flooding. Numerous approaches are available for formation pore pressure evaluation, however, these measurements become a challenge inside a cased borehole, and a list of possible options is short: either the casing is to be perforated, or the production tubing needs to be disconnected to perform the pressure tests. We present a method for through-casing evaluation of formation pore pressure without shutting down production. We suggest equipping an observation well with a borehole gravimeter and acquiring time variations of the vertical component of the gravity field. Changes in gravity occur during gas production and are related to time variations of formation pore pressure. Gravity changes obtained in the observation well are supposed to be inverted for time-dependent formation pore pressure variations beyond the casing. Our results and recommendations are based on numerical modeling of pore pressure spatial distribution during gas field exploitation and relevant changes in borehole gravity. Benchmark models were elaborated in order to consider a dynamic process of pressure changes in time and space under conditions similar to those in the Medvezhye gas field (Russia). Different modeling scenarios are considered for early and late stages of gas field exploitation. The sensitivity analysis was performed to estimate quantitatively a sensitivity of borehole temporal gravity changes to variations in formation pore pressure behind the casing. Based on resolution analysis we justify the possibility to extract the gravity measurements directly related to changes in pore pressure from the total changes in the gravity field due to reservoir exploitation. The impact of pore pressure on the gravity field measured in boreholes during the water flooding is also evaluated, and obtained results are discussed.

4D inversion of borehole gravity data for monitoring fluid fronts

SUMMARY Monitoring fluid movement is an important component in enhanced oil recovery (EOR) and CO2 sequestration. The newly available slim-hole gravimeter operating at high temperature offers a new avenue for such monitoring efforts because of the direct sensitivity to the change in the density distribution. We present a time-lapse gravity inversion algorithm for recovering the front of injected fluid using borehole gravity measurements. We assume that the horizontal extent of the fluid can be represented by a polygon with known but variable thickness and density contrast due to fluid substitution. We represent the evolution of the front as a 4D function of the spatial position and time since the initiation of the injection. The inversion can be carried out either independently at discrete time points or as a single inversion simultaneously over all time points. We demonstrate that the latter approach is superior in that it is more stable and offers improved capability in detecting break-through events at later times. In this paper, we will describe the details of the two inversion approaches, including two different model objective functions in polar coordinates and the nonlinear solution strategies. We will illustrate the advantages and drawbacks of independent and simultaneous 4D inversions using numerical examples

Interpretation of borehole gravity data of the Lalor volcanogenic massive sulfide deposit, Snow Lake, Manitoba, Canada

We have acquired borehole gravity data along five drillholes intersecting the Lalor volcanogenic massive sulfide deposit hosted in the eastern Flin Flon greenstone belt at Snow Lake, Manitoba, Canada. Inverted apparent interval density (IAID) logs were calculated from the borehole gravity data and compared with lithofacies and [Formula: see text] logs; the latter of which is a geochemical proxy for differentiating volcanic rocks of felsic to mafic composition. The IAID anomalies predominantly reflect alternating mafic and felsic volcanic rock units in the footwall and hanging wall of the massive sulfide deposit. IAID lows are associated with [Formula: see text] highs that correspond to rhyolite and rhyodacite intervals in the hanging wall. IAID lows with associated [Formula: see text] peaks in the footwall occur within intervals of gneiss and schist formed by metamorphism of hydrothermally altered rocks, suggesting that these IAID lows still reflect the felsic composition of their volcanic protoliths. A significant peak-to-peak Bouguer anomaly of 0.66 mGal caused by an estimated excess mass of 0.7 mT can be correlated with gamma-gamma density signature of the main sulfide ore zone in three boreholes. This anomaly is aligned with the ore zone after restoring the displacement along a northeast-dipping structure. When integrated with drillhole lithology and lithogeochemistry logs, gravity borehole data can, in addition to the direct detection of mineralization, be used as a subsurface geological mapping tool.

Minimum-structure borehole gravity inversion for mineral exploration: A synthetic modeling study

A borehole gravimeter for the diameters of holes typically used in mineral exploration has recently been developed. Investigating how the data from such instruments can contribute to the gravity interpretation procedures used in mineral exploration is therefore appropriate. Here, results are presented from a study in which synthetic data for 3D exploration-relevant earth models were inverted and the impact of borehole data assessed. The inversions were carried out using a minimum-structure procedure that is typical of those commonly used to invert surface gravity data. Examples involving data from a single borehole, from multiple boreholes, and combinations of borehole and surface data, are considered. Also, a range of options for the particulars of the inversion algorithm are investigated, including using a reference model and cell weights to incorporate along-borehole density information, and an [Formula: see text]-type measure of model structure. The selection of examples presented demonstrates what one can and cannot expect to determine about the density variation around and between boreholes when borehole gravity data are inverted using a minimum-structure approach. Specifically, the density variation along a borehole can be accurately determined, even without constraints in the inversion, but this capability decreases dramatically a few tens of meters from a borehole.

Time-lapse gravity: A numerical demonstration using robust inversion and joint interpretation of 4D surface and borehole data

There are a number of ongoing developments in the 4D gravity method for time-lapse production and sequestration problems. Complex model construction is an essential component for meaningful feasibility studies and data interpretation in 4D. In the case of oil reservoirs, for example, the 4D gravity method must deal with very small density contrasts at depth, and overly simplistic model representations of a field site may not properly guide monitoring efforts for effective reservoir management decisions. The tracking of injected fluid through inversion can be significantly improved with the joint interpretation of surface and borehole gravity data. This is particularly relevant for time-lapse gravity problems where subtle changes in density contrast at depth may not produce surface data that alone contain the necessary information to recover the boundaries of fluid movement. Complex model construction and robust inversion show that time-lapse gravity surveys may contribute to improved production efficiency and reservoir management in-between the more traditional, and expensive 4D seismic surveys. Results hint that the true value of time-lapse gravity as an additional tool for production efficiency and reservoir management may be greatly undervalued. Data simulated within current and ongoing sensor technologies, combined with advances in computing power and robust inversion, can extract much more meaningful information about fluid movement in reservoirs that are geometrically complex and relatively thin and deep. To illustrate these points, simulations are performed using a representation of the published Jotun Field in the Norwegian North Sea, a well-studied site demonstrating successful application of the time-lapse seismic method, and reconstructed for 4D gravity understanding.

The vision thing

Editor's note: This collection of thoughts about the future of gravity and magnetics arose from an email discussion between Pat Millegan and Alan Reid. Reid, an active member of the SEG G&M Committee, has been instrumental in helping the community agree on data transfer standards after 30 years of “discussion.” Their email chat prompted Millegan to solicit comments from others about the future of the G&M disciplines. All thoughts and comments were written for inclusion in this article. Where are the potential-fields geophysics methods going? My crystal ball is as cloudy as anyone's, but the profession is seeing an upsurge of innovation and I'm seeing some of my young man's visions turning into reality instead of old man's dreams (Joel 2:28). The changes in data resolution and sensitivity are making completely new applications possible, so that potential-fields methods are expanding from regional tools into prospect scale and to some extent to reservoir scale. In the next 10 years, I believe that we will see more than 75% of the following objectives achieved. 1. Satellite gravity data will take over completely in the offshore regional environment and will be used routinely in planning all offshore seismic surveys. 2. Marine and land gravity acquisition will go to higher resolutions and sensitivities and will remain bread-and-butter work. 3. Conventional airborne gravity will become available at higher sensitivities, better resolution, and lower prices. 4. Vector gravity data will be collected from moving platforms (land, sea, and air). 5. Gravity tensor gradient measurement will become widely used, notably for reservoir problems, and will be available from multiple contractors using more than one instrument type. Their increased sensitivity will permit them to penetrate the minerals sector. 6. Borehole gravity data will (finally) find widespread use in the reservoir environment. 7. Absolute gravity and gravity gradient surveying will become routine both on the ground …

Interpretation of borehole gravity data from the German continental deep drilling project : Wolfgram, P A; Casten, U; Gatto, H; Neubauer, F M; Richter, J E T; Strack, K M Sci DrillingV3, N1/3, 1992, P70–75

Interpretation of borehole gravity data from the German continental deep drilling project : Wolfgram, P A; Casten, U; Gatto, H; Neubauer, F M; Richter, J E T; Strack, K M Sci DrillingV3, N1/3, 1992, P70–75

Frequency domain expressions for surface and borehole gravity potential due to two- and three-dimensional mass models

Frequency domain expressions for surface and borehole gravity potential due to selected two- and three-dimensional mass models with linear variation of density are derived using conjugate complex variables formulation. The mass models include elliptical bodies, vertical and horizontal polygonal prisms, polyhedral bodies and bodies with rotational symmetry. Such analytic expressions are essential for development of interpretative theory for surface and borehole gravity data in the Fourier domain.

A New Field Experiment in the Greenland Ice Cap to Test Newton's Inverse Square Lawa

A geophysical experiment was conducted in a 2-km-deep hole in the Greenland ice cap at depths between 213 m and 1673 m to test for possible violations of Newton's inverse-square law. A detailed ice-sounding radar survey was carried out to 5 km from the hole and merged with regional airborne radar data to define the basement interface. Highly accurate gravity measurements were carried out to 15 km from the hole to detect lateral density changes in the bedrock. The measurements were controlled with very accurate satellite and conventional positioning techniques. The basement interface model was input into a Newtonian computation to correct the observed gravity for known earth structure resulting in an anomalous gravity gradient of +3.87 mGal. A 3 dimensional ideal body analysis of the surface and borehole gravity data provided a means of bounding all possible Newtonian solutions for lateral density variation below the ice. Solutions with regional gravity offsets greater than or equal to10 mGal and density contrasts less than or equal to0.30 g/cm/sup 3/ are possible. We cannot unambiguously attribute the anomalous gradient to a breakdown of Newtonian gravity because there remains the possibility it is due to unexpected geological features in the rock below themore » ice. 11 refs., 6 figs., 1 tab.« less

Simultaneous interpretation of surface and borehole gravity data from the Siljan area

Simultaneous interpretation of surface and borehole gravity data from the Siljan area

Focusing on SEG Continuing Education: Borehole gravity surveys

Can accurate density and porosity be determined for important drillhole intervals where conventional well logs perform poorly, where old casings shield rocks from standard evaluation methods, or where irregularly distributed porosity (e.g., fracture or vugular porosity) defies conventional assessment? Can porous zones not penetrated by the drill but within production distances of the well be detected? Can the extent or proximity of subsurface structural features or stratigraphic units be determined from borehole measurements? Under what conditions are these achievable goals of a borehole gravity (BHG) survey? Can repeated surveys made during the lifetime of a petroleum reservoir provide useful information about reservoir mass changes? What information can be extracted from the integrated analysis of BHG data, conventional well logs, and other geophysical and geological information? What are the limitations of BHG surveys?

Reexamination of the values used as constants in calculating rock density from borehole gravity data. Short note : Robbins, S L Geophysics, V46, N2, Feb 1981, P208–210

Reexamination of the values used as constants in calculating rock density from borehole gravity data. Short note : Robbins, S L Geophysics, V46, N2, Feb 1981, P208–210

Reexamination of the values used as constants in calculating rock density from borehole gravity data

The U.S. Geological Survey has a research program in bore‐hole gravimetry (Mast, 1978, p. 17). As part of this program, I have been examining the accuracy of constants F and k used in the formula for determining in‐situ bulk density from borehole gravity data.

Terrain effects of cultural features upon shallow borehole gravity data

Borehole gravity surveys of oil and gas reservoirs are usually run at depths of thousands of feet, and such surveys are not likely to be significantly affected by the ordinary works of man. However, borehole gravity surveys for engineering applications and the evaluation of mineral deposits and aquifers may have maximum depths of only a few hundred feet and, therefore, are more likely to be affected by the gravity fields of man‐made surface features such as basement excavations, quarries, gravel pits, sewage plants, road fills and cuts, storage piles, mine dumps, tank farms, etc.

Effects of terrain on borehole gravity data

The effect of terrain on gravity measurements in a borehole and on formation density derived from borehole gravity data is studied as a function of depth in the well, terrain elevation, terrain inclination, and radial distance to the terrain feature. The vertical attraction of gravity [Formula: see text] in a borehole resulting from a terrain element is small at the surface and reaches an absolute maximum at a depth of about one and one‐half times the radial distance to the terrain element, then decreases at greater depths. The effect of terrain on calculated formation density is proportional to the vertical derivative of [Formula: see text] and is maximum at the surface, passes through zero where |[Formula: see text]| is greatest, and reaches a second extremum of opposite sign to the first and of much lower magnitude. Accuracy criteria for borehole‐gravity terrain corrections show that elevation accuracy requirements are most stringent for a combination of nearby terrain features and near‐surface gravity stations. Sensitivity to terrain inclination is also greatest for this combination. The measurement of the free‐air gradient of gravity, commonly made’slightly above the ground surface, is extremely sensitive to topographic irregularities within about 300m of the measurement point. The effect of terrain features 21.9 to 166.7 km from the well [Hammer’s (1939) zone M through Hayford‐Bowie’s (1912) zone O] on calculated formation density is nearly constant with depth. At these distances, the terrain correction will be equivalent to a dc shift of about [Formula: see text] of average elevation above or below the correction datum. The effect of topography beyond 166.7 km is not likely to exceed [Formula: see text].

Interpretation of borehole gravity surveys in a native-sulfur deposit, Culberson County, Texas

Borehole gravity surveys to determine formation densities were conducted by the U. S. Geological Survey in four exploratory wells penetrating the native-sulfur deposit at the Duval Culberson property, Culberson County, Texas. The borehole gravity meter has a depth of penetration comparable to the recovery radius of the Frasch process for mining sulfur, which is an advantage for evaluating the sulfur content of heterogeneous deposits. Mineralized zones are treated as three-component systems--bioepigenetic limestone, interstitial sulfur, and water-filled pores. A ternary diagram relating combinations of these components to borehole gravity densities, together with empirical data relating sulfur content and water-filled porosity, are used to estimate sulfur volumes from the borehole gravity data. Estimated sulfur contents range from 0 to 33 percent. Comparisons with the amount of sulfur observed in core and cuttings from the four study wells and from surrounding wells show both the positive benefits of a greater range of investigation and the negative effects of relying solely on formation density as a measure of sulfur volume. Estimates of sulfur content based on borehole gravity data can supplement conventional analyses of core, as well as standard geophysical surveys of boreholes.