A Novel Workflow for Geosteering a Horizontal Well in a Low Resistivity Contrast Anisotropic Environment: A Case Study in Semoga Field, Indonesia

This paper describes the combination of strategies deployed to optimize horizontal well placement in a 40 ft thick isotropic sand with very low resistivity contrast compared to an underlying anisotropic shale in Semoga field. These strategies were developed due to previously unsuccessful attempts to drill a horizontal well with multiple side-tracks that was finally drilled and completed as a high-inclined well. To maximize reservoir contact of the subject horizontal well, a new methodology on well placement was developed by applying lessons learned, taking into account the additional challenges within this well. The first approach was to conduct a thorough analysis on the previous inclined well to evaluate each formation layer’s anisotropy ratio to be used in an effective geosteering model that could better simulate the real time environment. Correct selections of geosteering tools based on comprehensive pre-well modelling was considered to ensure on-target landing section to facilitate an effective lateral section. A comprehensive geosteering pre-well model was constructed to guide real-time operations. In the subject horizontal well, landing strategy was analysed in four stages of anisotropy ratio. The lateral section strategy focused on how to cater for the expected fault and maintain the trajectory to maximize reservoir exposure. Execution of the geosteering operations resulted in 100% reservoir contact. By monitoring the behaviour of shale anisotropy ratio from resistivity measurements and gamma ray at-bit data while drilling, the subject well was precisely landed at 11.5 ft TVD below the top of target sand. In the lateral section, wellbore trajectory intersected two faults exhibiting greater associated throw compared to the seismic estimate. Resistivity geo-signal and azimuthal resistivity responses were used to maintain the wellbore attitude inside the target reservoir. In this case history well with a low resistivity contrast environment, this methodology successfully enabled efficient operations to land the well precisely at the target with minimum borehole tortuosity. This was achieved by reducing geological uncertainty due to anomalous resistivity data responding to shale electrical anisotropy. Recognition of these electromagnetic resistivity values also played an important role in identifying the overlain anisotropic shale layer, hence avoiding reservoir exit. This workflow also helped in benchmarking future horizontal well placement operations in Semoga Field. Technical Categories: Geosteering and Well Placement, Reservoir Engineering, Low resistivity Low Contrast Reservoir Evaluation, Real-Time Operations, Case Studies

A Study on the Electrical Characteristics of Fractured Gas Hydrate Reservoirs Based on Digital Rock Technology

The electrical characteristics of fractured gas hydrate reservoirs were investigated through the diffusion-limited aggregation model, digital rock technology, and the finite element method. The results show that the fracture and gas hydrate have a significant effect on the electrical characteristics of rock partially saturated with gas hydrate, where the matrix pore and fracture mixed gas hydrate form a dual-porosity system. Due to the fracture and gas hydrate effect, the electrical characteristics of fractured gas hydrate reservoirs cannot be described well by traditional Archie equations. The resistivity index vs. water saturation curve of fractured gas hydrate reservoirs shows a nonlinear relationship for different gas hydrate pore habits (pore-filling, cementing, and grain-coating types), and this curve consists of two parts with different gas hydrate saturation exponents for pore-filling and cementing gas hydrate and presents a curve without a fixed water saturation exponent for grain-coating gas hydrate. Fractured gas hydrate reservoirs with different fracture apertures, different gas hydrate pore habits, and saturation features will lead to macroscopic electrical anisotropy. The results of theoretical analysis and numerical simulation show that the electrical anisotropy coefficient of fractured gas hydrate reservoirs is a function of gas hydrate saturation. The function curve consists of three segments with the turning point for pore-filling and cementing gas hydrate, and this curve can be divided into two parts through the turning point. The findings of this study can help for a better understanding of the electrical characteristics of fractured gas hydrate reservoirs, which have great significance for the exploration and development of gas hydrate resources.

Magnetotelluric study in the Los Lagos Region (Chile) to investigate volcano-tectonic processes in the Southern Andes

To analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data-processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1 s and two-dimensional for periods greater than 10 s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80–100 km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.

Magnetotelluric study in the Los Lagos Region (Chile) to investigate volcano-tectonic processes in the Southern Andes

In order to analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1s and two-dimensional for periods greater than 10s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80 - 100km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.

Conductivity inversion of unidirectional CFRP laminate based on ECT using neural network

For the electrical anisotropy of carbon fiber reinforced polymer (CFRP), conductivity of unidirectional CFRP laminate in three directions was inverted in this paper. The three-dimensional eddy current electromagnetic model of unidirectional composites was constructed by ANSYS software, and the influence of the electrical conductivity of the material on the detection signal of the probe in the longitudinal, transverse and thickness directions was studied. In order to improve the amplitude of the probe output signal induced by the change of conductivity, the optimal detection angle of the eddy current probe was determined. On this basis, the relationship between the conductivity and the detection signal was studied to estimate the initial values of the conductivity based on the experimental data obtained by the eddy current testing (ECT). According to the forward model, the theoretical probe voltage under the estimated conductivity were calculated. The database consisting of conductivity and corresponding theoretical results was built for the neural network to construct the mapping that can estimate conductivity by experimental results. Using neural network for iteration, the conductivity was inverted quickly and precisely.

Magnetotelluric study in the Los Lagos Region (Chile) to investigate volcano-tectonic processes in the Southern Andes

In order to analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1s and two-dimensional for periods greater than 10s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80 - 100 km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.

Novel Approach on Thin Bed Reservoir Case Study from Muda Formation, Natuna Basin

The presence of shale in thin beds reservoirs affects formation evaluation where the standard conventional log analyses are not designed to properly correct this effect. The conventional logging tools, with low vertical resolution, are not able to characterize these thin beds. This implies that log values do not represent the true bed or layer properties, but rather an average of multiple beds. Muda Formation are characterized by thin bed layers, made up of clastic rock sequences with dominant lithology of sandstone inter-bedded with shale, siltstone, and organic material as confirmed by drilling cuttings, logs response, and also supported by observation from sidewall cores. There are many uncertainties related to the presence of thin beds, primarily sand, silt, shale or their combination in term of their petrophysical properties and lateral extent. Inadequate reservoir characterization can cause significant amounts of oil and gas to remain unidentified. Accurate petrophysical parameters determination play an important role in the development plan of a field. The lateral and vertical variations in the petrophysical properties of the reservoir lead to different scenarios of the field development. The study of Muda Formation in this structure has integrated the sidewall core and log data. The contribution of the thin sand laminae to the average log response resulted in underestimating the porosity (Ф) and hydrocarbon saturation (Sh). The advanced measurement, like the resistivity anisotropy, proved quite useful as the vertical and horizontal resistivity across these beds leading to measurable electrical anisotropy. The resistivity measured perpendicular to the bedding is significantly higher than resistivity measured parallel to the bedding. The situation occurs due to high resistivity sand layers interbedded with low resistivity shale layers. The true sand porosity and hydrocarbon saturation were calculated using the laminated sand shale sequence and calibrated with core data. The study led to the more realistic petrophysical estimation of the sand shale laminae. A combination and integration of high-resolution image log for sand count, nuclear magnetic resonance (NMR) for porosity evaluation and triaxial resistivity for volumetric model through Laminated Sand Analysis approach are found useful to solve thin bed reservoir issue.