Engineered Solution to Reduce the Impact of Lost Circulation During Drilling and Cementing in Rumaila Field, Iraq

Abstract

Over the last 60 years in the oil and gas industry, lost circulation has been the major nonproductive time (NPT) issue worldwide during well construction (12% of total NPT). According to industry figures, more than USD 2 billion are spent combating circulation loss each year. In the Rumaila field of Iraq, the NPT due to lost circulation is significant, estimated to be 46% of the total NPT recorded. The Rumaila oil field is a super-giant oil field in southern Iraq; the field is estimated to contain 17 billion barrels, which accounts for 12% of Iraq's oil reserves. In the Rumaila field, at least 45% of the wells experience severe to total circulation loss at depths of 450 to 650 m true vertical depth (TVD) while drilling in the karst facies of the Dammam formation. Numerous strategies, requiring up to 24 days of rig time or 15 cement plugs in some extreme cases, have been applied by different service companies without consistent results. This has resulted in higher operating costs and delayed production. Some of the identified reasons for the low success rate are poor wellbore characterization, weak assessment of the problem, and improper execution. The foundation of the proposed engineering and operational approach is the characterization of the wellbore with the caliper and formation image logs for better understanding of fractures and estimation of fracture volume. The approach also includes two cement slurry-type options—an increased slurry rheology with reduced density and a slurry with rapid gel strength development. The proposed approach is made complete with the engineered estimation of cement slurry volume, based on loss rate, and proper field deployment. Developed on the basis of previous experiences and optimization of hydrostatic pressure, cement slurry rheology, and gel strength development, the engineered lost circulation solution significantly improves the lost circulation control results. Extended evaluation of cement plug jobs with the engineered design and proper execution has proven the effective impact of the solution, curing losses during the drilling phase (<3 days). This solution has proven to be an effective and key element, contributing to the increased success rate of the subsequent primary cementing operation and overcoming the challenges of zonal isolation.

Modeling Lost-Circulation into Fractured Formation in Rock Drilling Operations

“…Because of the nature of this problem that requires immediate intervention, there is a need in the industry to establish an accurate and efficient modeling tool [13]. Table 1.…”

“…Another study for 103 wells in the Duvernay area in Canada reported a loss of $2.6 million and 27.5 days of NPT because of lost circulation (Figure 2). In the Middle East, a group of 144 wells in Rumaila field, Iraq, encountered major lostcirculation problems [13], causing 48% of all drilling issues and loss of 295 days NPT, as illustrated in Figure 3. This high occurrence of lost-circulation was attributed to the presence of conductive natural fractures in the carbonate formations.…”

A review of technological advances and open challenges for oil and gas drilling systems engineering

“…It is estimated that over 2 billion USD is spent annually in combating and mitigating the problem of lost circulation. 27 Overbalanced drilling (in which the well pressure is higher than the formation pressure) is a common practice in the Brazil oil industry. This practice, coupled with the occurrence of naturally fractured reservoir zones further induces drilling mud loss.…”

Drilling of Horizontal Wells in Carbonate Reservoirs of Middle East for Petroleum Production – Investigation of Hydraulics for the Effect of Tool Joints

“…The velocity across the annulus is calculated using the below Equation 9: V annulus = q/(2.448 × (d 2 2 -d 1 2)) (9) where q is the flow rate of the fluid. Plastic viscosity (μp) is the difference in between 600 rpm and 300 rpm readings of variable speed rheometer as presented in Equation 10; μ p = θ 600 -θ 300 (10) The yield point (τ y ) is calculated using the relation given in Equation 11; τ y = θ 300 -μ p (11) For each wellbore section a critical velocity (v c ) is required to be calculated using Equation 12; v c = [1.08μ p + 1.08 (μ p 2 + 9.26(d 2 -d 1 )2(μ p )ρ) 0.5 ]/ [ρ(d 2 -d 1 )] (12) where ρ is the density of the drilling fluid.…”