Figure 1. Wells drilled using MAX-BRIDGE technology in Saudi Arabia. (Figures courtesy of Baker Hughes Drilling Fluids)

As fields mature, permeable formations can become depleted, often leading to problems related to lost circulation and differential sticking. When faced with depleted formations, there exists a dilemma over how to best address the borehole stability and well control requirements of higher pressured zones while mitigating the risks of differential sticking or mud losses from the depleted zones. In these wells, there is little or no operating margin between pore pressure and formation breakdown. The hydrostatic pressure required to stabilize higher pressured formations (like native pressured shale) is often higher than the pore pressure in depleted formations, leading to problems of differential sticking and lost circulation.

It is likely that before the onset of depletion, these same formations could have been drilled in one hole section. Depressurization of these fields over time does occur and often creates the need for intermediate casing strings in order to manage these pressure variations.

Advanced bridging technologies

A cost-effective solution to this quandary was developed in the form of a novel bridging package that helps address issues related to highly depleted formations. Conventional bridging theories, modeling software and field techniques based on pore throat size are available and routinely used to develop effective strategies. However, field evidence suggests that problems can still occur when dealing with unknown pore throat sizes and fractures or when trying to bridge a formation with a very heterogeneous pore throat distribution. Baker Hughes Drilling Fluids’ MAX-BRIDGE customized bridging technology was developed to help address the unpredictable nature of some formations using a combination of highly durable yet deformable bridging materials. Their deformable nature allows the materials to re-shape to fit undetermined pore throats and mold themselves into micro-fractures. This ability to “mold to fit” was previously unattainable with conventional bridging technology and techniques.

The approach taken with the customized bridging system is based on a combination of two advanced bridging technologies developed by Baker Hughes Drilling Fluids. The first is an inert, naturally deformable sealing polymer, which molds into pore spaces and micro-fractures typically found in shale or low-porosity sand. As a result, pore pressure transmission levels are reduced, thereby mechanically stabilizing the formation.

The second is a finely graded, highly resilient synthetic graphite which deforms under pressure and bridges larger pore throats or fractures typically found in higher permeability sand or limestone formations. The combined effect is to reduce sticking tendency and reduce volumes of mud losses to weaker formations while stabilizing shale and mitigating the risk of differential sticking with higher mud weights. The graphite additive acts also acts as a mechanical lubricant, thereby contributing to torque and drag reduction in deviated wells.

Field experience

Application of the customized bridging approach has been pioneered in the field by Saudi Aramco, who used the technology to successfully address a multitude of drilling problems over the variably pressured carbonates and sands in the Harad field in the Eastern Province of Saudi Arabia.

Carbonate reservoirs in the Harad field are characterized by a series of variable pressures in the Khuff formation. While Khuff C formations typically require mud densities in the range of 10.6 to 10.9 lb/gal for borehole stability and well control, the higher pressure Khuff D formation requires a minimum fluid density of 11.7 lb/gal. These formations must be drilled at a near horizontal orientation to maximize reservoir contact and potential for production. This requirement leads to a high risk for differential sticking and downhole losses to the depleted zones as the minimum mud density is limited by a requirement to stabilize the higher pressured zones.

Offset wells in the Harad field had experienced drilling difficulties. Differential sticking had occurred while drilling the main hole, resulting in significant expense due to lost time, lost tools and the need to side-track the well.

For the sidetrack well, the fluids service company recommended the addition of the customized bridging technology to the existing mud to provide additional sealing, stabilizing and lubricating attributes to the trouble zone. The sidetrack well had an 88? inclination with a measured total depth of 12,889 ft (3,931 m).

The Khuff C formation was drilled with fluid at 10.9 lb/gal with no indication of differential sticking or mud losses. No stability problems were encountered during the higher pressured drilling of the Khuff D formation at 12.0 lb/gal fluid density. After re-entry into the depleted Khuff C formation, there was no differential sticking or mud losses. The subsequent trip out of hole was slick, and the casing was run and set at planned depth without any problems.

The use of the customized bridging technology enabled the operator to drill 1,700 psi overbalanced without experiencing any downhole mud losses, borehole stability problems or differential sticking. The use of the system resulted in significant reductions in non-productive time and equipment costs as compared to the original hole.

Following on from this success, the approach has been adopted by Aramco in its Qatif and Ghazal fields as well as an increasing interest in global applications in South Texas and the Gulf of Mexico. To date, more than 89 wells have been drilled in Saudi Arabia using the customized bridging system in formations such as the Unayzah A and B; the Jauf; the Jilh; the Minjur; the Sudair; the Arab C and D; and the Khuff A, B, C and D. (Figure 1). The maximum overbalanced drilling pressure achieved with the system to date is 3,761 psi, and the longest interval drilled has been 9,640 ft (2,940 m, Figure 2). The customized bridging technology is also applicable from the surface formation to deeper depths and formations. It is fully capable of bridging formations of different geological ages and geology starting from lower Cretaceous to lower Devonian age. Additionally, the bridging products have been used in various water-based and oil-based systems.

The use of the customized bridging technology has broad applications in drilling depleted and high permeable zones in gas wells, and also it is an excellent precautionary tool to drill wildcat and exploratory oil wells. Adding the customized bridging technology to an existing water-based or oil-based system increases the rock stress cage that will allow operators to achieve optimum flow rate and increased rates of penetration.