Comprehensive Studies of Formation Water for Achimov and Bazhenov Formations — Revitalizing Archived and Old Cores

Abstract

The sediments of Bazhenov (BF) and Achimov (ACh) formations have been studied for more than 50 years, and to date, a large amount of core material obtained during drilling at the stage of exploration is stored in the core stores of Russian industrial companies. The rise in the cost of the complex of geological exploration and the emergence of new research methods enabled studies of the old/dry core from previously drilled exploration wells. BF and ACh reservoir rocks have low permeability and initially low water saturation, and therefore, during storage, rock samples could partially retain pore water. To study its composition and quantitative content, we used the previously proposed integrated approach with proven effectiveness in fresh low-permeability BF rocks. The studied materials include BF rock samples from 3 different fields stored for 2 years after sampling in the laboratory, and ACh rock samples from 12 fields, cored more than 10 years ago at the geologic exploration stage. The complex technique includes determining free and bound water contents by the evaporation method with isotopic analysis (δ18O and δD) of the released water. The modified water extraction method provides a range of pore water salinity, while the modified alcohol ammonium chloride method measures the cation exchange capacity (CEC). Auxilary methods include Rock-Eval pyrolysis to determine organic matter content, X-ray diffraction analysis (XRD) to assess the mineral composition of sediments. The verification of the results for the BF rock samples was carried out according to the data obtained for the new/fresh core, while for the ACh rock samples — according to the results of the direct chemical composition analysis of the formation water. Despite the low permeability, we found that the BF and ACh rock samples during storage lost almost all (up to 90%) free water due to evaporation. At the same time, salts from formation water remained in the pore space, which made it possible to estimate the range of its NaCl salinity: 1.84–14.7 g/L for ACh rock samples and 4.49–20.19 g/L for BF rock samples. The obtained values set the lower limit of the possible pore water salinity of the studied ACh and BF rock samples. Moreover, the results match those from direct salinity measurements in the ACh depth intervals and the results of fresh BF core laboratory studies. We showed that the old/dry BF and ACh core could be effectively used to assess the bound water content with subsequent determination of its genesis from isotopic composition data and CEC measurements. This opportunity relies on the high clay content in the studied BF and ACh rock samples. The obtained results show the fundamental capability of informative laboratory studies of BF and ACh rock samples from public and private core storage facilities and give new life to archived and old/dry core.