Deep Drilling Into Khuff Formation
Authors: Samhouri F.A. , Al-Chalabi A. 
Published: December 2007

The onshore exploration for gas in the Khuff formation of Abu Dhabi started in 1980. Abu Dhabi Co. for Onshore Oil Operations (ADCO) has drilled two wells to a total depth (TD) of 20,363 ft [6207 m] in Bu Hasa field and to 21,022 ft [6408 m] in Jam Yaphour area, and other wells are in progress. Highly pressurized zones and high bottomhole temperatures (BHT's) have been of major consideration in both drilling and completion of these wells.

This paper deals with the design of the first two wells and the modifications incorporated in the design of the third well. It also describes briefly the techniques used in drilling the wells with emphasis on bits, coring, drilling fluid, logging, and cementing.

Introduction

Large gas reserves have been discovered in the Khuff (or equivalent formations) offshore U.A.E. as well as in other Persian Gulf countries, such as Qatar, Saudi Arabia, Bahrain, and Iran.

A 1979 ADCO study on the geology of the Permian Age in its concession area showed significant Khuff prospects.

Two wells have been drilled by ADCO into the Khuff in its onshore concession. Another well is being drilled it is in its 8 1/2-in. [216-mm] phase-to explore the Khuff formation in the Bab field and two wells are planned to be drilled in the coastal areas of ADCO's concession (Fig. 1).

The two wells drilled reached a TD of 20,363 ft [6207 m] in Bu Hasa field and 21,022 ft [6408 m] in the Jam Yaphour area just south of the city of Abu Dhabi. BHT's were greater than 400 deg. F [204 deg. C].

The two wells drilled reached a TD of 20,363 ft [6207 m] in Bu Hasa field and 21,022 ft [6408 m] in the Jam Yaphour area just south of the city of Abu Dhabi. BHT's were greater than 400 deg. F [204 deg. C].

The first two wells were designed with a 10,000-psi [69-MPa] pressure rating for the casing scheme and wellhead configuration. Experience gained in drilling the first two wells showed the presence of high-pressure zones and high BHT's. Therefore, the design of the third well has been modified to include 15,000-psi [103-MPa] high-temperature casing and wellhead configuration.

Both rotary and turbodrilling techniques were used in these wells but turbodrilling proved to be the best technique.

Water-based drilling fluid was used for drilling to TD. Logging and cementing operations were carried out successfully at the elevated BHT.

Geological Formations

A brief description of the geological formations is given in Table 1. The formations down to the Jurassic Age have been drilled and evaluated by ADCO. Information concerning formations of the Triassic and Permian Ages was obtained while drilling the Khuff wells.

Khuff Well Design

Two Khuff wells were drilled and a third is in progress.

The first Khuff well (Bu Hasa) was designed according to the information collected from the Khuff wells drilled in the area. This well was drilled to 20,363 ft [6207 m]. Experience gained while drilling the first well was used to modify the well design of the third Khuff well (Bab).

The second well (Jam Yaphour) was originally designed as a Jurassic well with TD of 14,500 ft [4420 m]. At a later stage, the well was deepened to the Khuff formation. TD reached in the second well was 21,022 ft [6408 m]. This well was deepened at the same time the first well was drilled. Although the Khuff was reached, the design is not recommended.

Design of First and Third Khuff Wells

One of the main factors in selecting the design of casing sizes and points is the desirability of 8 1/2-in. [216-mm] hole through the prospective Khuff formation with 7-in. [178-mm] casing. The other casing points are governed by the characteristics of the formations above the Khuff.

The advantages of 8 1/2-in. [216-mm] hole and 7-in. [178-mm] casing through the Khuff formation are summarized in the following.To have a contingency for drilling below production casing or to set an extra casing string in case of a major problem while drilling, the smallest practical sizes are 5 7/8-in. [149-mm] hole and 4 1/2-in. [114-mm] casing.To achieve good drilling progress at the Khuff formation depth, the hole size should be suitable for efficient transmission of hydraulic and mechanical energy.An 8 3/8-in. [213-mm] hole is the smallest size to give full 4-in. [102-mm] core size required for adequate formation evaluation by core analysis.The majority of logging tools give an optimal performance in the 8 1/2-in. [216-mm] hole. Logging in small holes also increases the possibility of stuck tools, which is serious when allowable overpull is restricted because of depth.Operating at these depths involves high stresses on the drilling equipment, and smaller sizes are more prone to failure than are larger sizes. Also, fishing operations are restricted by slim hole and the loss of sensitivity resulting from reduced weight of downhole component and hole drag.

JPT

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