A Novel Data Processing Method for Array Induction Logging in Tight Carbonate Reservoirs Drilled With Oil-Based Mud
Authors:   PetroChina Research Institute of Petroleum Exploration & Development, Li Chaoliu , Yuan Chao ,   PetroChina Oil & Gas and New Energy Company, Feng Zhou ,   PetroChina Research Institute of Petroleum Exploration & Developmen, Tian Han ,   PetroChina Research Institute of Petroleum Exploration & Developmen, Wang Lei ,   China University of Petroleum (East China), Wang Songran ,   PetroChina Research Institute of Petroleum Exploration & Development
Published: November 2024

The targeted reservoir, which is referred to as the Sinian Dengying Formation in the central Sichuan Basin, is a typical tight carbonate sediment rich in vugs, and it features low matrix porosity with Rt 200 Ω·m, even 2,000 Ω·m at some specific sections. This sediment is often buried below a layer of gypsum, so many wells are drilled with oil-based mud ensuring borehole stability. It leaves no choice of resistivity measurements other than the array induction tool. However, the signal-to-noise ratio (SNR) of raw conductivity signals is very low, with negative values existing for these coils with short T-R spacing. As a result, array Rt logs with different depths of investigation (DOI) from the classic “software focusing” algorithm overlaps and flattens out, which is too poor for fluid typing and quantifying the reserve. This paper presents a novel weak conductivity signal processing method, which significantly improves the accuracy of array Rt logs, and provides an effective way to the evaluation of such complex carbonate gas reservoirs. This novel method includes several steps: (1) estimating the lower limit of conductivity based on the Archie equation supposing that formation is gas saturated, (2) careful selection of the raw signals with better SNR from these coils with longer spacing (the signal of short spacing is seriously affected by oil-based mud (OBM)) and making some appropriate correction to avoid negative value, (3) building an inclined layered initial model (depends on real geometry) and forward modeling the theoretical array induction signals based on its specification of these chosen coils, comparing the modeled signals with the actual signals, adjusting the model and repeating the comparison until the best fit to get the equivalent conductivity, and (4) finally, the resolution matching algorithm, which is commonly used in the industry, is applied to these equivalent logs with different T-R spacing and array Rt logs with a designated resolution, and DOI is produced. Moreover, by setting the horizontal and vertical resistivity, respectively, in the initial model, the array Rh and Rv logs with given RES and DOI can be extracted respectively at tilted electrical anisotropic formation. The traditional algorithm combines the weak signal of all coils and produces distorted array Rt logs. The new method yields more reasonable results by optimizing signal input and performing appropriate corrections. More importantly, the invasion characteristics of Rt logs with different DOIs are kept and shown more clearly, which is very useful for fluid-type interpretation. As a typical case, DLL and HRLA tools are introduced. After that, OBM is replaced by WBM at Well A. HRLA gives information about the invasion profile. The comparative analysis shows that this new method provides a proper resistivity logs output with a closer result to that of RLLD at shallow-invasion intervals. In addition, the invasion profile from different DOIs becomes clearer and serves as an indication of fluid typing, which is validated by an oil test at several wells.