Logging-while-drilling Resistivity Research Articles

A study on directional resistivity logging-while-drilling based on self-adaptive hp-FEM

Numerical simulation of resistivity logging-while-drilling (LWD) tool response provides guidance for designing novel logging instruments and interpreting real-time logging data. In this paper, based on self-adaptive hp-finite element method (hp-FEM) algorithm, we analyze LWD tool response against model parameters and briefly illustrate geosteering capabilities of directional resistivity LWD. Numerical simulation results indicate that the change of source spacing is of obvious influence on the investigation depth and detecting precision of resistivity LWD tool; the change of frequency can improve the resolution of low-resistivity formation and high-resistivity formation. The simulation results also indicate that the self-adaptive hp-FEM algorithm has good convergence speed and calculation accuracy to guide the geologic steering drilling and it is suitable to simulate the response of resistivity LWD tools.

Response simulation and theoretical calibration of a dual-induction resistivity LWD tool

In this paper, responses of a new dual-induction resistivity logging-while-drilling (LWD) tool in 3D inhomogeneous formation models are simulated by the vector finite element method (VFEM), the influences of the borehole, invaded zone, surrounding strata, and tool eccentricity are analyzed, and calibration loop parameters and calibration coefficients of the LWD tool are discussed. The results show that the tool has a greater depth of investigation than that of the existing electromagnetic propagation LWD tools and is more sensitive to azimuthal conductivity. Both deep and medium induction responses have linear relationships with the formation conductivity, considering optimal calibration loop parameters and calibration coefficients. Due to the different depths of investigation and resolution, deep induction and medium induction are affected differently by the formation model parameters, thereby having different correction factors. The simulation results can provide theoretical references for the research and interpretation of the dual-induction resistivity LWD tools.

Fracture orientation and induced anisotropy of gas hydrate-bearing sediments in seismic chimney-like-structures of the Ulleung Basin, East Sea

During drilling in 2010 in the Ulleung Basin, East Sea, Logging-While-Drilling (LWD) was conducted for thirteen sites. LWD data shows various characteristics indicating presence of gas hydrate-bearing sediments. In particular, a series seismic chimney sites are characterized by anomalous log data (i.e. high resistivity and velocity values), compared to surrounding marine sediments. At chimney sites, the resistivity and velocity log values are over 200 Ω-m and 3000 m/s, respectively. Moreover, log values of low density (less than 1.1 g/cm3) indicating the presence of massive hydrates also correlate with intervals with the highest resistivity and velocity. Gas hydrates at the seismic inferred chimney sites occurred within inclined fractures in the mud dominated sediments. These gas hydrate-filled fractures were identified on LWD resistivity images and X-ray images of pressure cores. The gas hydrate-filled fracture intervals coincide with high measured resistivity intervals at three sites (Sites UBGH2-3, UBGH2-7, UBGH2-11). In most cases, high measured resistivity translated into high hydrate saturations via Archie's relationship; but the high saturations derived from the Archie's relationship appear to overestimate gas hydrate saturations when compared to pressure core samples and acoustic log measurements. Also, in intervals with inclined gas hydrate-filled fractures (e.g., UBGH2-3), there is notable separation between phase-shift and attenuation resistivity logs, with 2 MHz resistivity measurements being significantly higher than 400 kHz resistivity measurements. In this study, the analysis of fractures with dip angles greater than 30° on the resistivity log-images show dip angles between 43 and 63° in average for the three sites examined. The dip azimuth of the fractures at Sites UBGH2-3 and UBGH2-7 dominate westerly to southwestern direction, while the fractures at Site UBGH2-11 are characterized by no preferred orientation. This fracture pattern indicates that maximum horizontal stress direction at the time of fracture formation was not constant. In addition, the differential compaction of the sediments after deposited may be contributed to the development of fracture.

Wired-Drillpipe Field Trials Reveal Potential Benefits Over Traditional Pipe

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 163560, ’A Summary of Wired-Drillpipe Field Trials and Deployments in BP,’ by Stephen T. Edwards, Chris J. Coley, Nick A. Whitley, Richard G. Keck, SPE, Vishwahnath Ramnath, Tommy Foster, Keith Coghill, and Mark Honey, SPE, BP, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5-7 March. The paper has not been peer reviewed. Field trials were conducted by BP on the commercial version of wired pipe with a comprehensive suite of logging-while-drilling (LWD) tools, measurement-while-drilling (MWD) functionality, and rotary-steerable- capability on two Wyoming wells in 2007. Since then, BP has deployed wired pipe commercially in more than 14 wells at four additional locations (Trinidad, North Sea, Colombia, and deepwater Gulf of Mexico), representing a good cross section of drilling conditions and challenges. Basics of How Wired Pipe Works Wired drillpipe incorporates a section of coaxial cable in each joint. The cable passes through a gun-drilled hole in the connection on each end of the joint. Between connections, the cable is encased in a strong stainless-steel sheath, which is held in tension within the inner diameter of the pipe adjacent to, but not bonded to, the inner wall. Figs. 1 and 2 show schematics of the wired pipe. Inductive coupling enables the signal to pass from one joint to the next, through ferrous coils that are embedded in the pin end of one joint of pipe and the opposing box end of the subsequent joint of pipe. These coils come very close to each other when the connection is made up, but, because the coupling is inductive, they do not need to touch, and insulating material such as pipe dope does not block the signal. However, over many joints of pipe, there is a loss of signal strength, and, for this reason, a battery-powered booster sub is deployed approximately every 1,500 ft. The booster assemblies also provide housing and a connection point for along-string measurements. On the bottomhole-assembly (BHA) end of the drillstring, wired pipe connects to the measurements providers and steering assembly through an interface sub, enabling two-way communication to the network, usually provided by the LWD provider. At the top of the drillstring, the network is connected to surface servers by a data swivel housed in the top drive. Wired-Pipe-Enabled Applications Real-Time Imaging. Several different LWD imaging tools are commercially available today. Two types of tools that have been run with wired pipe are the LWD azimuthal density imaging tool and LWD resistivity imaging.

Numerical simulation of LWD resistivity response of carbonate formation using self-adaptive hp-FEM

Most of the carbonate formation are highly heterogeneous with cavities of different sizes, which makes the prediction of cavity-filled reservoir in carbonate rocks difficult. Large cavities in carbonate formations pose serious threat to drilling operations. Logging-whiledrilling (LWD) is currently used to accurately identify and evaluate cavities in reservoirs during drilling. In this study, we use the self-adaptive hp-FEM algorithm simulate and calculate the LWD resistivity responses of fracture-cavity reservoir cavities. Compared with the traditional h-FEM method, the self-adaptive hp-FEM algorithm has the characteristics of the self-adaptive mesh refinement and the calculations exponentially converge to highly accurate solutions. Using numerical simulations, we investigated the effect of the cavity size, distance between cavity and borehole, and transmitted frequency on the LWD resistivity response. Based on the results, a method for recognizing cavities is proposed. This research can provide the theoretical basis for the accurate identification and quantitative evaluation of various carbonate reservoirs with cavities encountered in practice.

Numerical Simulation of a Multi-Frequency Resistivity Logging-While-Drilling Tool Using a Highly Accurate and Adaptive Higher-Order Finite Element Method

AbstractA novel, highly efficient and accurate adaptive higher-order finite element method (hp-FEM) is used to simulate a multi-frequency resistivity logging-while-drilling (LWD) tool response in a borehole environment. Presented in this study are the vector expression of Maxwell’s equations, three kinds of boundary conditions, stability weak formulation of Maxwell’s equations, and automatichp-adaptivity strategy. The newhp-FEM can select optimal refinement and calculation strategies based on the practical formation model and error estimation. Numerical experiments show that the newhp-FEM has an exponential convergence rate in terms of relative error in a user-prescribed quantity of interest against the degrees of freedom, which provides more accurate results than those obtained using the adaptiveh-FEM. The numerical results illustrate the high efficiency and accuracy of the method at a given LWD tool structure and parameters in different physical models, which further confirm the accuracy of the results using the Hermes library (http://hpfem.org/hermes) with a multi-frequency resistivity LWD tool response in a borehole environment.

Interpretation of porosity and LWD resistivity from the Nankai accretionary wedge in light of clay physicochemical properties: Evidence for erosion and local overpressuring

In this study, we used porosity to assess the compaction state of the Nankai accretionary wedge sediments and any implications for stress and pore pressure. However, hydrous minerals affect porosity measurements, and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state. The water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale. We used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals. The results indicate that several apparent porosity anomalies are significantly reduced by this correction, implying that they are in part artifacts from hydrous minerals. The correction also improves the fit of porosity estimated from logging‐while‐drilling (LWD) resistivity data to porosity measured on cores. Low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion, and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity‐effective stress relationships of the sediments to a reference curve. Additionally, a comparison of LWD data with core data (resistivity and P wave velocity) obtained at Site C0001 landward of the mega‐splay fault area, suggested a contribution from the fracture porosity to in situ properties on the formation.

Integrated analysis of well logs and seismic data to estimate gas hydrate concentrations at Keathley Canyon, Gulf of Mexico

Accurately detecting and quantifying gas hydrate or free gas in sediments from seismic data require downhole well-log data to calibrate the physical properties of the gas hydrate-/free gas-bearing sediments. As part of the Gulf of Mexico Joint Industry Program, a series of wells were either cored or drilled in the Gulf of Mexico to characterize the physical properties of gas hydrate-bearing sediments, to calibrate geophysical estimates, and to evaluate source and transport mechanisms for gas within the gas hydrates. Downhole acoustic logs were used sparingly in this study because of degraded log quality due to adverse wellbore conditions. However, reliable logging while drilling (LWD) electrical resistivity and porosity logs were obtained. To tie the well-log information to the available 3-D seismic data in this area, a velocity log was calculated from the available resistivity log at the Keathley Canyon 151-2 well, because the acoustic log or vertical seismic data acquired at the nearby Keathley Canyon 151-3 well were either of poor quality or had limited depth coverage. Based on the gas hydrate saturations estimated from the LWD resistivity log, the modified Biot–Gassmann theory was used to generate synthetic acoustic log and a synthetic seismogram was generated with a fairly good agreement with a seismic profile crossing the well site. Based on the well-log information, a faintly defined bottom-simulating reflection (BSR) in this area is interpreted as a reflection representing gas hydrate-bearing sediments with about 15% saturation overlying partially gas-saturated sediments with 3% saturation. Gas hydrate saturations over 30–40% are estimated from the resistivity log in two distinct intervals at 220–230 and 264–300 m below the sea floor, but gas hydrate was not physically recovered in cores. It is speculated that the poor recovery of cores and gas hydrate morphology are responsible for the lack of physical gas hydrate recovery.

LWD for Imaging, Wellbore Placement, and Formation Evaluation

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17646, "LWD for Imaging, Wellbore Placement, and Formation Evaluation," by J.C. Rasmus, SPE, C. Esmersoy, SPE, J. Seydoux, and A. Hawthorn, Schlumberger, prepared for the 2005 Offshore Technology Conference, Houston, 2-5 May. Logging-while-drilling (LWD) measurements are widely available for a variety of applications. Many of the first generation LWD measurements are similar to those obtained traditionally with wireline methods. However, there are measurements available now that are either complementary or are entirely unique compared with traditional wireline measurements. Introduction LWD borehole-seismic measurements can enable placing the bit on the seismic map or producing a seismic-impedance map ahead of the bit while drilling the well. These measurements provide accurate depth determination of drilling targets and identification of pore-pressure transgressions. LWD ultradeep-resistivity (UDR) measurement technology meets the requirements for geosteering at the reservoir level by use of subseismic resolution. Its major applications include delineation of the oil/water contact (OWC) and distance to reservoir caprock and bottom shale. Well Placement Since the emergence of LWD in the 1980s, the industry wanted a reliable while-drilling borehole-seismic measurement. However, several technical roadblocks prohibited its realization until recently. These difficulties included the nature of and required accuracy of the measurement, timing issues, sensors, automation, and the lack of an electrical connection between the downhole tools and the surface in a while-drilling configuration. Technological solutions to these road-blocks have since been found. Seismic-while-drilling data are delivered in real time to make critical decisions while a well is being drilled. They also replace intermediate wireline check-shot surveys, eliminating the need to stop the drilling process and pull drilling equipment out of the hole for the duration of a survey. Real-time accurate positioning of a horizontal borehole within a reservoir can be performed with standard LWD tools. However, a limitation of these measurements is the relatively small volume of investigation with traditional LWD sensors, which typically probe no deeper than 1 m into the formation. Therefore, unpredictable geological events are, in many cases, detected too late to allow proactive geosteering decisions. A prototype LWD resistivity tool was developed with a much larger radial response that allows detection of lithology features and fluid contacts that are tens of meters away from the borehole. The tool has a modular design with separate transmitter and receiver subassemblies that can be placed a large distance away, anywhere in the bottomhole assembly (BHA). The main output is estimated distance to boundaries indicated by resistivity contrasts. These distances are computed through inversion of a formation model in real time. On the basis of tool measurements, determining the optimum formation model and its related parameters allows proactive geosteering decisions and pro-vides significant geological insight into the reservoir structure.

Combining Pulsed Neutron Spectroscopy and LWD Resistivity for Analysis Behind Casing in Small Holes

Summary Formation analysis from behind casing is gaining more relevance in assessing reservoirs, not only in aging fields but also in new wells completed either without the full complement of openhole petrophysical measurements or with openhole measurements of inadequate quality. Before this technology was available, formation evaluation in freshly drilled holes was dependent on the availability of openhole log measurements. This paper discusses field examples in which reservoirs were evaluated without a full suite of openhole log measurements. Examples from two sidetracks in a mature North Sea field supported by water injection are presented. The small hole size limited openhole data acquisition to resistivity and gamma ray (GR) measurements, which were acquired using logging-while-drilling (LWD) technology. In both cases, pulsed neutron (PN) measurements were recorded through casing to augment the existing data. Porosity and formation sigma (macroscopic thermal neutron capture cross section) measurements were obtained. Fluid saturations were computed using this porosity and the LWD resistivity. Saturations also were computed from the carbon/oxygen measurement, and these agreed with the resistivity-based answers. Lithology analysis was performed using elemental spectral capture yields and was reliable and consistent fieldwide. The results show that, in a mature oil field, PN measurements can provide a complete lithology and saturation analysis in circumstances in which insufficient openhole petrophysical measurements are available.

The Use of Resistivity-at-the-Bit Images and Annular Pressure While Drilling in Preventing Drilling Problems

Summary The cost of well construction can exceed budget dramatically if drilling operations are plagued by wellbore-instability problems and excessive time and resources are needed to free stuck pipe, regain circulation, or clean the hole efficiently. Annular pressure while drilling has been recognized recently as one of the key downhole measurements for aiding real-time diagnosis of the condition of the wellbore and drilling fluids. In addition to conventional drilling-mechanics measurements, time-lapse logging-while-drilling (LWD) documents the dynamic change in measured properties during well construction. This can be essential in diagnosing wellbore failure, and the mechanism of failure, in addition to dynamic processes such as cuttings buildup. An example from a horizontal well in the North Sea is used to illustrate how LWD resistivity images can be used in conjunction with annular-pressure-while-drilling measurements to detect the onset of drilling-induced fractures. Although the majority of azimuthal images have been acquired to understand the geology and petrophysics of reservoirs, the images usually contain features resulting from geomechanical processes. Analysis of these features reveals important information for optimizing drilling and understanding the geomechanics of the well. Coupling resistivity images to the pressure measurement enables problem identification and correct remedial actions to optimize the drilling operation, for example, ensuring that swab and surge pressures are kept to a minimum, and correct hole-cleaning procedures are used to prevent irreversible formation breakdown.

Resistivity Measurements at the Bit Provide Real-Time Formation Evaluation Before Invasion

Real-time formation evaluation with today's conventional horizontal drilling techniques is limited by the distance between the bit and resistivity measurements. Logging-while-drilling (LWD) sensors reach the formation long before wireline measurements, and so generally view it before wellbore degradation, but not before some invasion has occurred. Rapid invasion, called spurt, may mask true resistivity in some formations. The solution to this problem is to relocate logging measurements to the bit itself. A new LWD resistivity tool improves and simplifies formation evaluation by allowing geologists to visualize and log the formation around the wellbore before mud invasion or wellbore damage has occurred. The tool is normally run as a near-bit stabilizer on a rotary bottomhole assembly or just above the motor in a steerable assembly. It makes five formation evaluation resistivity measurements and an azimuthal gamma ray measurement. One resistivity measurement uses the bit as part of the measuring electrode to provide the earliest possible indication of a change in formation resistivity. This real-time correlation capability leads to rig time savings during searches for casing or coring points, in effect allowing immediate "geostopping." The top of a reservoir can be cored without danger of drilling up the interval first. At-the-bit resistivity also allows the fastest detection of pore pressure anomalies. Four additional resistivity measurements are high-resolution focused electrode resistivities. One measurement uses a ring electrode to make an azimuthally averaged resistivity that is accurate up to 20,000 ohm-m. The other three use button electrodes to make azimuthal resistivity measurements that scan the borehole as the tool rotates. Together the ring and three button electrodes give four depths of investigation with identical vertical resolutions. Conventional 2-MHz LWD resistivity measurements are limited to environments favoring induction-type settings; that is, resistive mud (i.e., fresh or oil-based mud) and conductive rock. The solution to this problem is a high-resolution, focused electrode resistivity that provides accurate Rt measurements for the first time in "laterolog" environments of high formation resistivities and salty mud. An example is carbonates, where 2-MHz tools are inherently less accurate. This high resistivity measurement has excellent vertical resolution, allowing evaluation of beds as thin as 3 in. and detection of beds as thin as 1 in. Multiple depths of investigation of the button measurements, in combination with the focused electrode resistivity measurement, allow evaluation of permeable beds from early-time invasion detection. The array of three button electrodes provides azimuthal resistivities that scan the borehole as the tool rotates. These measurements form the basis for advanced processing and interpretation of full-bore resistivity images (see Fig. 1), introducing the potential for fracture detection, dip determination, and improved formation evaluation and characterization. The technical leap that allows measurements to be made at the bit is a wireless telemetry system that sends data from sensors near the bit to the measurement-while-drilling (MWD) tool up to 200 ft. behind the bit, bypassing the intervening drilling tools. Data sent to the MWD system are then transmitted to the surface in real time using mud-pulse telemetry. Resistivity at the bit is measured by attaching the tool directly to the bit and driving an alternating electric current down the collar, out through the bit, and into the formation. The current returns to the drillpipe and drill collars above the transmitter. In water-based mud, returning current is conducted from the bit, through the mud, into the formation, and back to the bottomhole assembly. In oil-based mud, which is an insulator, current returns through the inevitable but intermittent contact of the collars and stabilizers with the borehole wall, leading to a qualitative indication of resistivity. Formation resistivity is obtained by measuring the amount of current flowing into the formation from the bit and normalizing it to the transmitter voltage. Although drilling with at-the-bit measurements is still in its infancy, petrophysical measurements near the bit provide numerous benefits for geologists, petrophysicists, and drillers and extend the range of conditions under which it is possible to measure formation resistivity accurately while drilling. SI Metric Conversion Factors ftx3.048* E-01=m in.x2.54* E+00=cm *Conversion factor is exact.