Overpressure zone under the Krishna–Godavari offshore basin: geophysical implications for natural hazard in deeper-water drilling

Abstract

Accurate knowledge of pore pressure is fundamental to any safe and economic well construction. Here, we present results that are indicative of over pressure zones (OPZ) for five wells drilled under the Krishna-Godavari offshore basin (KGOB) at the Eastern Continental Margin of India (ECMI). OPZ in areas of crustal flexuring can act as potential geohazard while drilling. These wells locate at water depths of 515-1,265 m, where their penetrated-vertical-depth reaches up to 3,960 m in clastic sediments. pore pressure gradient (PPG) and fracture pressure gradient (FPG) are estimated from acoustic log for all five wells, while the Normal Compaction Trend (NCT) and pore pressure are calculated from Miller's sonic equation. Top of OPZ is indicated by values that are higher than the NCT; departure from NCT is observed at depth intervals of 1,320-2,180 m, 1,700-3,960 m, 1,600-1,880 m, 1,420-2,609 m and 2,080-2,200 m for the respective Wells 1 through 5. The pressure data from Modular Dynamic Tester (MDT) agree well with the pore pressure values obtained from the logs. The Overburden Gradient (OBG), PPG and FPG values increase rather slowly with total depth in deeper-water of KGOB when compared to the wells located in shallow water depth. Consequently, the operating safety margin between PPG and FPG decreases as the water depth increases, and this clearly leads to an increase in the number of casing strings to reach the target depth. Certain basic conclusions on the potentiality of natural hazard for drilling operations are drawn on the basis of these results, but evidently, further studies are warranted to present a more composite picture of OPZ under KGOB.

Overpressure Zones in Relation to In Situ Stress for the Krishna-Godavari Basin, Eastern Continental Margin of India: Implications for Hydrocarbon Prospectivity

“…For the benefit of safe deep-water drilling in offshore KG-B, it is therefore of largely significant that accurate estimation of PPG, FPG and in situ stress is made in order to diminish geohazards which may occur during drilling. These findings are also corroborated by geological evidences on the Pliocene environments in KG-B which are interpreted to be comparable to the modern upper continental slope with rather large scale mass-transport deposits and submarine canyons (Chatterjee et al 2011). For instance; Shanmugam et al (2009) consider that tsunamis, earthquakes, frequent tropical cyclones, shelf-edge canyons with steep-gradient walls and sea-floor fault scarps as favorable factors for triggering submarine mass movements.…”

“…Accordingly, we defined the NCT graphically and its deviation in travel time from sonic logs for the wells in KG-B for the purposes of delineation of OPZ. We have reported it before that the overpressure in the KG-B is most likely related to its low permeability sediment and is confined by low permeability media (Chatterjee et al 2011). NCT represents best possible fitted linear data in the low permeable zone, like shale (Fig.…”

“…The pressure estimate from the Eqs. (2) and (3) had already been compared to the pressure measured by RFT and MDT tools for onshore and offshore wells respectively (Singha and Chatterjee 2014;Chatterjee et al 2011). Previous author e.g.…”

Detection of overpressure zones and a statistical model for pore pressure estimation from well logs in the Krishna-Godavari Basin, India

“…Delineation of OPZ in the K‐G Basin indicates the increase of PP gradient from onshore to coastal areas. The development of OPZ toward offshore direction is in close agreement with the previous PP studies of K‐G offshore wells [ Chatterjee et al ., ]. The observed abnormal PP gradient in seven wells ranges from 11.85 to 13.10 MPa/km, whereas FP gradient varies from 17.40 MPa/km in normal pressured sediment to 19.78 MPa/km in overpressured sediments these wells.…”

“…This process is referred to as undercompaction or compaction disequilibrium and is being understood overpressure mechanism to explain and quantify overpressure in the K-G Basin [Chopra and Huffman, 2006;Roy et al, 2010]. Previously, Chatterjee et al [2011] describe that the high-rate deposition and transportation of sediment under ECMI is responsible for developing of overpressure in which excess fluid gets trapped in the low-permeable formation such as Vadaparru Shale in the east Godavari subbasin and in the offshore basin also. For delineation of OPZ for 10 wells under study, deviations from normal compaction trend (NCT) in travel time from sonic logs have been observed for seven wells on onshore K-G Basin.…”