Exploration and development of Heavy oil fields with high water cut and sand production in Muglad basin in Northern Africa started with vertical wells and as time progressed matured into drilling Horizontal wells.
Typically drilling challenges in this area include drilling very reactive shales , shallow kick off depths and high build rates, unconsolidated sand stones interbedded with shales which are sensitive to mud weight and are prone to lost circulation.
First few horizontal wells were drilled with conventional technology of positive displacement motor with silicate mud. Many of these wells faced hole cleaning issues leading to pack off ,excessive back reaming and stuck pipe incidences.Uneven build rates via sliding in interbedded formation leading to high borehole tortuosity . It is significant to note that due to these difficulties one of the planned horizontal wells was side tracked three times after stuck pipe incidences and finally completed as a 30 degree deviated well with a total cost over run of 300% above AFE.
Since then Rotary steerable system has been deployed to drill these challenging wells with significant improvement in drilling performance ,saving days and cost and eliminating stuck pipe incidences. This paper compares the performances of drilling with PDM Vs RSS in the same reservoir and presents the lessons learnt. A cost benefit analysis has also been performed and it clearly shows that RSS is both technically and economically a sound approach to drilling horizontal wells in Muglad basin.
Horizontal well drilling campaign in Sudan was started in 2004 with the following objectives:
• Increase well bore exposure to reservoir and hence increase the rate of production of heavy oil.
• Decrease the near well turbulence and hence decrease sand production.
• Decrease of Draw down pressure which will eventually lead to decrease in water cut.
The candidate wells were chosen in very well developed fields targeting by passed oil. Presence of good number of close by vertical offset wells offered good geological control for well placement. On the drilling front there were lots of challenges that were encountered while drilling the horizontal wells. In this paper we will look into the evolution of drilling techniques from the first well to the recent wells and see how continuous adoption of new and fit for purpose technology has minimized drilling risks and lead to economical drilling of horizontal wells.
Drilling Challenges in Muglad basin
Drilling of horizontal wells require in depth knowledge about the formations in the basin. Clear understanding of problems posed by the formations will go a long way in mitigating the drilling risks.
Figure 1 shows the formation stratigraphy of muglad basin Tendi and Nayil Formations are predominantly made up of water sensitive shale formations, which at times lead to bit and stabilizer balling. The next formation below is the Amal massive sandstone. Amal sand can be abrasive in some areas and usually have issues of lost circulation; build up of well bore deviation. Below Amal sand stone is Ghazal and Zarqa formations. These are made of consolidated sand shale intercalations and do not pose any major drilling problems. Aradeiba formation that lies beneath Zarqa can be sub divided into two major bodies, the upper and the lower Aradeiba shales. Both these formations are strong water sensitive and result in borehole instability, tight spots, pack off even while drilling vertical wells. The lower Aradeiba also has some sand stone reservoirs embedded between the shale bodies which contain some promising reserves. Bentiu reservoir is primarily a sandstone reservoir, which has very low pore pressure gradient. Differential sticking is one of the major concerns in this formation.