Abstract

This paper describes the concept, initial field case studies, and observations to date on Deep Water Disposal (DWD) in one of Petroleum Development Oman's (PDO) larger mature oil assets. The concept of DWD by fractured injection on a large scale has been quite successful. Current concerns relate to the size of injected fractures, and pressure build-up within the disposal zone.

Initial simulation studies were carried out with dedicated inhouse produced water re-injection software to asses the size and containment of the induced hydraulic fractures as a function of relevant injection parameters such as water quality, injection rate, injection volume and injection depth. This was essential as fractures are not allowed to break through the cap rock shale layer of the disposal zone into the overlying sand body which has communication with shallow potable aquifers. Based on stress data (minifracs, logs) and simulations it was concluded that fracture containment within the disposal zone was only guaranteed if fractures were initiated of the order two hundred metres below the cap rock.

Simulations of produced water injection were carried out also to assess well injectivity in the long-term, and to estimate the required injection pressures for the injection pumps. The simulations indicated that no injectivity decline long-term would be expected to take place. Field observations to date are in line with these simulations. Currently, each DWD well is injecting ca. 15,000–20,000 m3/d of unfiltered produced water with wellhead pressures of approx. 140 bar.

Fracture monitoring programmes are currently being carried out. Hydraulic Impedance Testing (HIT) was applied to measure fracture dimensions. It was found to be restricted to small fracture sizes due to the insensitivity of the reflected wave amplitude in the applied frequency range. Improved methods to measure fracture size such as tiltmeter surveys and downhole geophones are currently under investigation. Pressure monitoring within the disposal zone via observation wells has shown little pressure increase to date. Characterisation of this pressure response is a key reservoir driver in deciding how much water could be disposed at each of the disposal sites.

1. Introduction

Petroleum Development Oman (PDO) currently produces 450,000 m3/d of water as a byproduct of its oil output of 135,000 m3/d. The volume of water has been increasing steadily over time and is predicted to rise to over 900,000 m3/d by the year 2009. The disposal of large volumes of production water presents PDO with serious environmental and economical challenges. In order to manage these challenges, the PDO Produced Water Management Plan was developed and agreed with the Government and private shareholders. The development and implementation of this plan have been 1 described in an earlier paper1.

In PDO, re-injection into the ‘deep’ subsurface aquifer is the preferred disposal option. Earlier trials to investigate matrix injection of untreated water demonstrated that injectivity declined rapidly and that a matrix disposal scheme without 1 cleanup was not viable1. Attention therefore focussed on deep disposal of untreated water under fracturing conditions. Currently, Deep Water Disposal (DWD) is taking place via large-scale re-injection of unfiltered production water into the aquifer leg of the main producing formation at a depth of around 1000–1500m TVD.

This paper describes the technical concept, initial field case studies, and observations to date of DWD in the Nimr field in South Oman. This field handles the largest volumes of production water in PDO. The current field injection capacity amounts to 120,000 m3/d of water, which is injected into 7 DWD wells. The production water volumes in Nimr are expected to rise to over 250,000 m3/d by the year 2009.

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