Abstract

In the South West of Weizhou Island, a region of growing importance in oil and gas production in South China Sea, drilling delays and suspension of wells prior to reaching the targets due to wellbore instability, have occurred frequently. The hard brittle shales rich in the formation play the key problematic role. As a result, economic benefits through the use of new drilling technologies are significantly reduced due to high additional drilling costs.

Conventional water-based drilling fluids didn't conquer the problematic formation. They cannot prevent filtration from invading formation effectively, thereby the shale hydration and pore pressure increase cannot be eliminated. Alternatively, oil-based drilling fluids can provide superb borehole stability. Especially formulated with the efficient sealing materials, they can be available in drilling the hard brittle shale formation. It had been proven by some successful cases in the South West of Weizhou Oilfield. However, the employment of oil-based drilling fluids is cause for increasing concern due to offshore environmental restrictions and expensive disposal costs.

An environmentally acceptable water-based drilling fluid, which is based on methylglucoside and silicate, is developed to conquer the hard brittle shale formation. It has been confirmed from numerous laboratory results that the MS drilling fluid exhibited perfect filtration loss control and inhibitive abilities and also can effectively seal microfractures by physical packing and chemical precipitation. It stabilizes the hard brittle shale by the same mechanism as did oil-based drilling fluid in preventing shale hydration, pore pressure increase and weakening of shale by effectively developing sufficient osmotic force to offset hydraulic and chemical forces acting to cause filtration flux into the hard brittle shale.

MS drilling fluid has given its first offshore field trial in Weizhou oilfield. The data from the pilot well has showed that the novel drilling fluid exhibited excellent inhibition and lubricity which approaches or even exceeds oil-based fluids.

Introduction

For several decades, wells drilled in Weizhou field, a region of growing importance in oil and gas production in South China Sea, had experienced serious hole-instability problems related to drilling fluids due to the highly reactive and hydrated hard brittle shales existed in the Weizhou and Liushagang formations which are characterized by complex geological structures, a lot of faults and fractured zones. They are typically crushed formations in which a large number of micro fractures exsited. Borehole can experience hole enlargement, tight hole, drilling fluid a great loss to the formation, poor hole cleaning as well as severe sloughing shale problems. Frequent wiper trips were required as a result of sticking and unsuccessful logging. Worst of all, the well control safe was ever challenged. The difficulties resulted in drilling accidents occurred frequently, thereby drilling costs roared upwards, as shown in Fig.1. The hard brittle shale plays a bottleneck in the development of the Weizhou offshore field.

Conventional water-based drilling fluids didn't conquer the problematic formation. Over the years, a number of types of water-based drilling fluids, such as KCl/polymer system, KCl/lime/polymer system, PHPA system, PEM system and KCl/Silicate system, were all employed. They all failed in getting gauged hole and alleviating sloughing, as shown in Fig.2 and Fig.3.

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