Lower Khuff Research Articles

Cased-Hole Standalone Evaluation Succeeds in the Absence of Openhole Data

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 202894, “Cased Hole Standalone Evaluation: Breaking the Barrier To Successfully Evaluate Challenging Deep Carbonate Reservoirs,” by Pradeep Menon and Carey Mills, ADNOC, and Suvodip Dasgupta, SPE, Schlumberger, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually from 9-12 November. The paper has not been peer reviewed. Accurate petrophysical evaluations (formation lithology, porosity, and water saturation) are essential in characterizing potential reservoir zones and estimating resources in place. Typically, these evaluations rely on acquisition of openhole logging measurements; however, this is not always possible. The complete paper outlines two examples from tight gas reservoirs in two separate fields offshore Abu Dhabi in which openhole data could not be acquired and petrophysical analysis was undertaken using cased-hole log data. These evaluations successfully identified gas-saturated porous intervals in each well, one of which was successfully flow-tested. Introduction A growing need exists to increase gas production in the UAE. As a result, specific gas-production targets have been mandated from development of currently undeveloped deep gas carbonate reservoirs such as the Permo-Triassic Khuff formation, the middle Jurassic Araej formation, and the Permian Pre-Khuff Unayzah and Berwarth formations. Recent appraisal wells have aimed at evaluating these reservoirs systematically by acquiring a good suite of openhole logs, cutting conventional cores, and conducting well-testing operations. These well data are combined with an evolving regional understanding to better assess and ultimately develop these complex formations. An accurate petrophysical evaluation requires the petrophysicist to develop a realistic evaluation of formation lithology, porosity, and water saturation. These parameters provide the foundations for further work such as static modeling stands, and they must be robust. The Upper Khuff is composed of dolomite occasionally grading to calcareous dolomite with minor interbeds of claystone and anhydrite. In core and cuttings, the dolomite in the uppermost section exhibits a grainstone texture with poor intercrystalline/intergranular porosity. The Lower Khuff is composed of very hard dolomite in part grading to calcareous dolomite, medium-to-dark grey-brown in places, with occasional very-fine-to-medium grainstone texture and very poor intercrystalline porosity. In this paper, the Upper and Lower Araej members are interpreted to have been deposited in an open, marine- circulation shelfal environment, while the Uweinat member is considered to have been deposited in a more- restricted circulation setting with-in a similar shelfal environment. The Barrier Openhole logging generally is the preference for formation evaluation because it represents the simplest environments and benefits from a comprehensive list of available measurements. The variety of tools and diversity of output data available make openhole log acquisition the gold standard for formation evaluation. However, in certain situations in which openhole logging is not possible because of borehole conditions (re-entry of old cased wells, wellbore instability, over-pressure), no option exists other than acquiring petrophysical data in a cased-hole environment.

Relationship between karstification and burial dolomitization in Permian platform carbonates (Lower Khuff — Oman)

Large breccia fabrics associated with karst constitute an important structure in massive limestone successions. The dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids, but subsequently breccia fabrics can also govern the migration of later fluids. Therefore, breccias are highly relevant features to capture for reservoir characterisation. Outcrop analogues for Lower Khuff units in the Middle East present in the Central Oman Mountains reveal brecciated fabrics up to 10s of metres in diameter. These brecciated units are closely associated with dolomite bodies of late diagenetic origin.Based on an integrated set of data, the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids. The exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids. Based on the composition of the clasts and matrix in the breccias, two dolomitization events are interpreted to have affected the succession, one prior to (early diagenetic [ED] dolomite) and one after brecciation (late diagenetic [DT2] dolomite). Dolomite of shallow burial origin (ED dolomite) was only observed as clasts within breccia and is much more frequent than late diagenetic (medium to deep burial) dolomite clasts. Thus, the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization. Late diagenetic replacive dolomite (DT2 dolomite) forms 90% of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization, but is rarely present as clasts. Stable isotope measurements [δ18O: −2.5‰ to −6‰ VPDB and δ13C: 2.9‰ to 4.8‰ VPDB] suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids. The dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities. Thus, early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the Oman Mountains, and potentially in other similar settings elsewhere.

Application of Horizontal Wells To Reduce Condensate Blockage in Gas/Condensate Reservoirs

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 130996, ’Application of Horizontal Wells To Reduce Condensate Blockage in Gas/ Condensate Reservoirs,’ by Nathan Miller, SPE, Hadi Nasrabadi, SPE, Texas A&M University-Qatar, and Ding Zhu, SPE, Texas A&M University, prepared for the 2010 SPE International Oil & Gas Conference and Exhibition in China, Beijing, 8-10 June. The paper has not been peer reviewed. Much of the world’s gas reserves can be found in gas/condensate reservoirs. These reservoirs, from a recovery and deliverability standpoint, can have significant differences from oil reservoirs. When the pressure, either in the wellbore or in the reservoir, drops below the dewpoint pressure, a condensate phase appears that reduces gas production significantly. In this study, horizontal wells were used in a giant gas/condensate reservoir (North field, Qatar) to reduce the condensate blockage. Critical questions are: What fraction of a horizontal well’s increased gas production results from the increased formation contact and what fraction results from reducing condensate blockage? Introduction The largest gas/condensate reservoirs include the Arun field in Indonesia; the Cupiagua field in Colombia; the Karachaganak field in Kazakhstan; the North field in Qatar, which borders with the South Pars field in Iran; and the Shtokmanovskoye field in the Russian Barents Sea. All of these fields have one thing in common—condensate blockage. Condensate blockage occurs when a liquid phase forms around the wellbore because the pressure falls below the dewpoint pressure. Production performance can decrease significantly if these condensate-banking effects are not understood at the beginning of field development. Retrograde gases exhibit a lower gas/oil ratio (GOR) limit of 3,300 scf/STB and an upper GOR limit of 150,000 scf/STB. Retrograde gases have stock-tank liquid gravities between 40 and 60°API and can be lightly colored orange, brown, or greenish or can be clear. The North field is a massive offshore gas/condensate reservoir with more than 900 Tcf of proven natural-gas reserves. The abnormally pressured field covers more than 6000 km2. As Fig. 1 shows, the North field adjoins Iran’s South Pars field, which has 280 Tcf of recoverable natural-gas reserves. The initial reservoir pressure was 5,300 psi, and the initial reservoir temperature was 220°F. The field produces from a carbonate reservoir called the Khuff formation (late Permian to early Triassic). The Khuff formation consists of four noncommunicating, highly stratified layers. Discovered in 1971, North field appraisal drilling occurred over the next 14 years to quantify the reservoir-gas accumulations and to determine the reservoir-fluid and geological characteristics. The Alpha Project began in 1991 to develop the field’s reserves by producing natural gas and condensate from the Lower Khuff. The first concession was awarded in 1993, the first delivery of condensate occurred in 1996, and the first delivery of liquefied natural gas took place in 1997.