Abstract

Operation availability, environmental safety, and storage capacity of the slurrified cuttings re-injection into a shallow, permeable formation off-shore Abu Dhabi was evaluated using a computer simulator for hydraulic fracturing.

For the study, Umm Er Radhuma formation of Z field is chosen as the disposal formation, meanwhile Rus formation is regarded as the cap rock over the formation for isolating injected fluid and solid from propagating to seabed. A Geomechanical model of the formations which includes stress regime, formation pressure, rock mass properties, and leak-off characteristic was established with available data from literatures, wire line logging, and water disposal history in the same formation.

The numerical model, which is developed by the authors, provides the estimation of injection pressure, fracture geometry and size, and storage volume of the slurry with a modeled injection schedules. The effect of solid particle concentration which may reduce pressure near fracture tip and cause screen off is taken into account for the fluid flow analysis and the determination of the fractrue geometry.

Introduction

Drilling waste disposal is one of the major issues to reduce environmental impact by drilling operations. Cuttings Reinjection (CRI) is a cost effective measure if the environmental safety and operation availability is guaranteed. The authors propose design basis in view of Geomechanics, with theoretical background, analysis scheme and a numerical tool, which are applicable for fields off-shore Abu Dhabi and other candidates of the waste disposal sites.

In the paper, state of art numerical method for the simulation is shown with the scheme of Geomechanical modeling for the formations off shore Abu Dhabi. The availability of cuttings reinjection is proved feasible by the study, with remained issues such as more accurate data acquisition for cap rock integrity using well and seismic data.

Objective

This simulation study has been done in order to evaluate the feasibility and the safety of the cuttings re-injection in offshore Abu-Dhabi. The following are the prior objectives for this study.

  1. To ensure the deposition ability of fractures and rock properties

  2. To evaluate the fracture propagation path and the integrity of the formation boundaries.

Selection of Injection Zone

The injection zone must possess the proper physical properties to be able to accommodate repeated injections of solid laden liquids. The candidate zone should be of sufficient thickness to readily accept the anticipated injected volumes. And the zone should also be able to effectively contain the injected wastes.

In the case of Z field, Umm Er Radhuma (UER) and lower Simsima (SIM) formations that are the stratums of dolomatic limestone with the formation top at around 2,600 ft, are assumed to be selected as the injection zone. Rus formation, which consists of anhydrite, is regarded as the cap rock with the relative hard and higher horizontal stress. It is known that softer formations are favored candidates for injection due to their ability to encase injected materials as the general rule. Though there might be a possibility that carbonate zone could be inadequate for solid accommodations due to its minute matrix and low leak-off characteristics. However UER is considered as a potential candidate with the continuous thickness of more than 1500 ft. The formation is also known to be a lost circulation zone in some cases, so low pore pressure in which high injectivity and low injection pressure is anticipated.

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