Abstract

Underbalanced drilling (UBD) is becoming one of the fastest growing technologies in the oil and gas drilling industry. Some of the benefits of this technology are (1) reduced formation damage, (2) opportunity for reservoir evaluation while drilling, (3) reduced loss circulation and differential pipe sticking, (4) extended bit life, and (5) higher penetration rates. As a whole, it significantly reduces total drilling and completion costs.

During this last decade, the need to use UBD in Hassi Messaoud Field, Algeria as an alternative to overbalanced drilling becomes apparent. Several zones in the field are depleted sandstone reservoirs and conventional drilling operations led to a significant mud loss in the subjected pay zones. Furthermore, the hard and abrasive characteristics of the reservoir make it difficult to drill in term of bit wears and rate of penetration. In 2001, Sonatrach (Algerian National Oil Company) embarked on a series of field projects to evaluate operational and economical benefits of UBD in the field. Two projects were conducted, the first was a vertical well, and the second was a horizontal well. Post-drilling analysis showed improved rate of penetration (ROP), longer bit life, and reduced formation damage and loss circulation. Overall, the total drilling time and cost were substantially reduced. This paper presents the results including the operational successes and failures of using the UBD technology.

Introduction

Underbalanced drilling (UBD) is quickly becoming an important technology in the oil and gas drilling industry as it responds to the current petroleum economic issues. It has in many cases increased profits, not only by generating additional revenue from the production realized during drilling, but also by reducing full drilling and completion cycles costs and providing access to reserves that would otherwise not be produced. Additionally, UBD can help to maximize profit by: reducing or preventing lost circulation; improving penetration rates; providing real-time reservoir information such as deliverability; eliminating stimulation costs; and improving well bore productivity as a result of reduced formation damage. UBD involves intentionally manipulating the bottom hole circulating pressure (BHCP) so that it will be less than the static reservoir pressure (Pr), as a result, formation fluids will flow into the well.

The UBD projects are often undertaken in fractured reservoirs, depleted formations susceptible to an excessive well bore damage and mud loss, and in medium to hard rock where drilling rate is a problem. The HMD field in Algeria is one of those cases. Between 2000 and 2002, Sonatrach (Algeria National Oil Company) embarked on a series of trials to assess the operational feasibility and commercial benefits of drilling underbalanced depleted oil production zones. Two UBD projects were undertaken, the first was a vertical well (OMO- 243) and the second was a horizontal well (OMJZ-502). Now, Sonatrach is planning to carry out more UBD projects in order to quantify the economic benefits of this technology.

RECENT EXPERIENCE

HMD field is the most important oil producing reservoir in Algeria. Discovered in 1956, it is an oval anticline trending northeast to southwest, parallel to a major fault zone (Figure 2)

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