ABSTRACT

Deep drilling of Permo-Triassic / Palaeozoic section in Abu Dhabi area is, generally, costly and risky: Many unforeseen problems were encountered while drilling resulted in that some wells have not achieved their objectives in evaluating the deep reservoirs. One of these problems was the high pressure formation salt water flow from Gulailab (Jilh), Lower Khuff and the Pre-Khuff formations. Ths phenomena is most likely due to the presence of anhydrite /shale seals trapping the high pressure salt water flow migrating from the underlying reservoirs. If the mud logging unit, the drilling crew and the geologist on the well site succeeded to observe the pressure seal and/or other warning signs of kicks, they will predict early a high pressure zone before facing the problem. Thus, the drilling crew can be alert to monitor the hole and take necessary precautionary measures to continue drilling the hole safely. The paper will discuss case history examples on the high pressure formation salt water flow (HPFSWF) in deep drilling, from offshore/onshore areas in AbuDhabi.

I. INTRODUCTION

Pronounced overpressured zones were encountered in many fields in Abu Dhabi, mainly while drilling the deep Permo-Triassic/Palaeozoic wells. The overpressure phenomenon is a complicated issue and is associated with many geological factors e.g. effect of depth on overpressure distribution, facies changes as well as reservoirs and seal properties.

The type of problems encountered due to overpressuring in the Gulailah/Jilh, the Khuff and the Pre-Khuff sections vary from one well to another, regardless of the location. Heavy drilling fluids were generally used and intermediate casings were usually set in these formations to overcome these problems.

The main obective of this paper is of two folds: to highlight drilling problems with case history examples from offshore and onshore wells (Fig. 1)and to attempt to find geolgical explanations for the following questions:

  1. Is the overpressure zones in the Abu Dhabi basin a regional or local phenomenon?

  2. What is the lateral and vertical extent of the overpressure systems within the basin?

  3. Does underpressure also occur within Abu Dhabi basin?

  4. How does overpressure relate to sedimentation, lithology and geothermal gradient?

  5. What are the causes of overpressure in Abu Dhabi basin?

Obviously, these questions are essential for hydrocarbon exploration and development in the Abu Dhabi basin.

II. DRILLING PART
A. PROBLEMS ENCOUNTERED WHILE DRILLING DEEP WELLS IN ABU DHABI AREA

About 27 deep wells have been drilled in Abu Dhabi Area below Triassic, until the end of 1995. Most of these wells faced one or more of the following problems: Drill string problems, fishing operations, formation problems, casing cementing failures, mud problems, casing problems … etc. One of the formation problems which is classified as the most potential, expensive and operationally difficult to handle is the HPFSWF encountered while drilling the following hole sections: Jilh/Gulailah, Khuff and Pre-Khuff. These problems occurred in six offshore wells and four onshore wells. Each well has one or more of HPFSWF problems. Some of these problems resulted in that the well primary and/or secondary objectives have not heen achieved in spite of the high cost and time spent on the problem. To illustrate the above problems, four problematic wells will be presented in this part named F-1, Y-1, U-1 and S-1. The problems will be discussed under three cases as follows: Case (1) Gulailah/Jilh formation, case (2) Lower Khuff (LK) section and case (3) Pre-Khuff (PK) section.

A. PROBLEMS ENCOUNTERED WHILE DRILLING DEEP WELLS IN ABU DHABI AREA

About 27 deep wells have been drilled in Abu Dhabi Area below Triassic, until the end of 1995. Most of these wells faced one or more of the following problems: Drill string problems, fishing operations, formation problems, casing cementing failures, mud problems, casing problems … etc. One of the formation problems which is classified as the most potential, expensive and operationally difficult to handle is the HPFSWF encountered while drilling the following hole sections: Jilh/Gulailah, Khuff and Pre-Khuff. These problems occurred in six offshore wells and four onshore wells. Each well has one or more of HPFSWF problems. Some of these problems resulted in that the well primary and/or secondary objectives have not heen achieved in spite of the high cost and time spent on the problem. To illustrate the above problems, four problematic wells will be presented in this part named F-1, Y-1, U-1 and S-1. The problems will be discussed under three cases as follows: Case (1) Gulailah/Jilh formation, case (2) Lower Khuff (LK) section and case (3) Pre-Khuff (PK) section.

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