Delineating a cased borehole in unconsolidated formations using dipole acoustic data from a nearby well

A potential application for single-well acoustic imaging is the detection of an existing cased borehole in the vicinity of the well being drilled, which is important for drilling toward (when drilling a relief well), or away from (collision prevention), the existing borehole. To fulfill this application in the unconsolidated formation of shallow sediments, we propose a detection method using the low-frequency compressional waves from dipole acoustic logging. For this application, we perform theoretical analyses on elastic wave scattering from the cased borehole and derive the analytical expressions for the scattered wavefield for the incidence of compressional and shear waves from a borehole dipole source. The analytical solution, in conjunction with the elastic reciprocity theorem, provides a fast algorithm for modeling the whole process of wave radiation, scattering, and reception for the borehole acoustic detection problem. The analytical results agree well with those from 3D finite-difference simulations. The results show that compressional waves, instead of shear waves as commonly used for dipole acoustic imaging, are particularly advantageous for the borehole detection in the unconsolidated formation. Field data examples are used to demonstrate the application in a shallow marine environment, where dipole-compressional wave data in the measurement well successfully delineate a nearby cased borehole, validating our analysis results and application.

A NOVEL METHOD FOR FORMATION DENSITY MEASUREMENT IN CASED WELLS

Density logging is a fundamental logging method of open-hole logging series. In cased wells, the formation is usually treated as a cylindrical multiple-layer model including borehole, casing pipe, cement, and original formation, producing more complicated logging response compared to open-hole wells. Several studies have indicated that it is feasible to measure formation density using a density logging tool in cased well under certain conditions, of which the key problem is to determine casing thickness, cement thickness and cement density. This study proposes a novel method to evaluate formation density behind casing using a three-detector density tool. Considering that the backscattered gamma is easily affected by the density behind the casing, especially when the casing thickness is small, a designed ratio of high and low energy window counts of the scattered gamma spectrum is used to calculate casing thickness, which improves the lower limit of the detection. After obtaining casing thickness, the cylindrical multiple-layer model of cased well can be simplified to a cylindrical three-layer model, consisting of cased borehole, cement, and the original formation. Subsequently, the analytical expressions of the thickness of the middle layer (cement) as well as the density of the outer layer (original formation) can be derived based on the logging geometric factor concept, G, which borrowing from electric logging. Consequently, the cement thickness and original formation density could be calculated independently. To demonstrate the feasibility of the proposed method, experimental data are measured in a group of calibration wells. The data processing results show that the proposed analytical model fit well with the experimental data. In addition, the application conditions of the novel method are discussed based on experimental data. Finally, several log examples illustrate that under suitable casing and cement conditions, the formation density calculated with the proposed method in cased borehole is in good agreement with the corresponding open-hole density.

Can Heating Induce Borehole Closure?

The current study shows that heating a cased borehole in low-permeability shale rock can induce plastic deformation, leading to the closure of the casing annulus and decreasing annulus connectivity. The thermally induced borehole closure is interesting for the field operation of plug and abandonment (P&A), as it potentially saves operation cost and time by avoiding cutting casing and cementing. Lab experiments and numerical simulations are implemented to investigate the thermally induced borehole closure. Pierre shale and a field shale are tested. The lab experiments are performed by heating the borehole wall in a 10-cm-OD hollow cylinder specimen. Here, a novel experimental setup is applied, allowing for measuring temperature and pore pressure at different radii inside the specimen. Both the experimental data and the post-test CT images of the rock samples indicate the rock failure by borehole heating, and under certain conditions, heating results in an annulus closure. The decrease of hydraulic conductivity through the casing annulus is observed, but this decrease is not enough to form the hydraulic-sealed annulus barrier, based on the results obtained so far. Lab-scale finite-element simulations aim to match the lab results to obtain poro-elastoplastic parameters. Then the field-scale simulations assess the formation of shale barriers by heating in field scenarios. Overall, (i) the lab experiments show that heating a borehole can increase the pore pressure in shale and hence induce rock failure; (ii) the numerical simulations match the experimental results reasonably well and indicate that the heating-induced borehole closure can sufficiently seal the casing annulus in the field-scale simulation.

Effect of perforation on borehole acoustic wave logging: Modeling and application

In hydraulic-fracturing design and oil-recovery efficiency prediction, quantitative evaluation of perforation and its penetration depth is often desired, for which an effective method needs to be developed. We have developed a perforated cased-borehole model for the field test configuration and simulated the elastic wave propagation in the model excited by a borehole monopole source using the finite-difference method. After a multivariate regression analysis, the simulation results yield a relationship for the perforation penetration depth versus the induced P-wave traveltime change and formation slowness. For field application purposes, borehole array acoustic logging with a perforated borehole model is simulated. The calculated waveform data, in conjunction with the above-mentioned relationship, yield an accurate estimate of the perforation penetration depth. A field perforation test is used to develop application of our method.

Cased borehole acoustic-wave propagation with varying bonding conditions: Theoretical and experimental modeling

Acoustic measurements in cased boreholes are important for cement-bond evaluation behind the casing. In conjunction with a recently developed acoustic-wave theory using slip-boundary modeling, we carried out an experimental study for different cement-bond conditions. Four different cased-hole models were constructed, where the interface between the casing and the cement, and that between the cement and the formation, are decoupled or partially bonded to simulate the different cement bond conditions. An acoustic system is placed in the borehole to measure extensional casing waves along the borehole. By extracting the attenuation and velocity of casing waves from the experimental data, the bonding conditions were analyzed and compared with the theoretical modeling. The results indicate that, compared with the free-pipe situation, the casing waves are attenuated when there is some degree of bonding (good or poor) between the casing and the formation. However, when the poor bonding occurs at the cement-formation interface, the casing wave indicates significant velocity reduction and dispersion, the degree of the velocity change varying with the bonding condition. This wave phenomenon is predicted by the slip-boundary modeling. By adjusting the slip-boundary parameters in the modeling, the experimental results can be quantitatively modeled. These results are also confirmed by cased-hole acoustic logging data examples. The theoretical model can therefore be used to interpret cased-borehole acoustic-wave measurements.