Systematic Use of Tidal Effects for Reservoir Appraisal and Well Integrity Monitoring in a Near-Coast Onshore Environment

Abstract

Abstract Tidal effects in wells have been observed for millennia and have been analysed since the 1970s, following the introduction of high-resolution pressure gauges. Tidal effects are usually more obvious offshore and many papers focus on offshore wells with large tidal signals. This paper demonstrates that pressure changes caused by ocean tides are detectable in near-coast land wells and that, with careful analysis and processing, provide valuable additional information for reservoir characterisation. This paper uses a set of observations from multiple near-coast land wells with extended pressure histories acquired as part of campaign of production and interference testing for reservoir characterisation purposes. These pressure records have variable tidal signal and data quality. A variety of methods have been published to smooth tidally influenced pressure data and extract or remove the tidal component. These different tidal filtering techniques are tested on real data, and a preferred data processing procedure is selected appropriate for the large quantity and variable quality of data in the study area. The methodology allows data from multiple wells to be quickly processed and consistently screened. This paper gives examples of tidal behaviour in selected wells and compares the results across multiple wells in the study area to show how identification of subtle tidal effects is useful for: Field appraisal of a saturated oil-rim with gas cap by examining differences in tidal amplitude in wells in different parts of the field. Estimation of critical gas saturation by comparing tidal amplitudes before and after an extended well test. Monitoring well integrity in a producing field by tidal analysis of annular pressures. The paper also recommends a reservoir surveillance programme to obtain useful tidal data in onshore wells. This paper will be useful to engineers attempting to find and interpret subtle tidal data, instead of simply removing it. It recommends a systematic approach for using tidal data in wells where tidal signals are small and/or gauge resolution is poor, based on experience with real data with variable data quality. It also provides a case study to show the value and practical application of a surveillance programme to identify tidal data in near-coast land wells.