Depleted reservoirs pose numerous technical challenges in both the construction and completion phases for wells in dozens of producing fields, often putting into question the economical viability of these fields. Wellbore instability, severe lost circulation, and stuck pipe are just a few of the problems encountered when drilling into these low-pressured reservoir formations.No area better illustrates the problems with depleted reservoirs than the Lake Maracaibo region. Water-wet sands that frequently triggered costly seepage losses and differential sticking typify many of these zones. Some contain microfractured sandstone formations where uncontrollable losses of whole drilling fluid previously were the norm rather than the exception. Others are characterized by laminated sand and shale sequences, which create the conditions for slow, dangerous, and unduly expensive drilling. Attempts were made with underbalanced drilling, but in addition to the extra time and equipment required, wellbore instability lead to failed well construction and thus seriously degrading project economics.Over the past two years, a specialized drilling fluid has being utilized to drill these depleted reservoirs in Lake Maracaibo. This fluid combines certain surfactants and polymers to create a system of "micro-bubbles" known as aphrons encapsulated in a uniquely viscosified system. These aphrons are non-coalescing, therefore creating a micro-bubble network for stopping or slowing the entry of fluids into the formation.The aphrons allow conventional drilling equipment to be used to successfully complete many reservoirs that previously would have been candidates for underbalanced drilling only. This paper describes the development and application of the specialized "micro-bubbles" or aphron-based drilling fluid for drilling depleted reservoirs by controlling downhole mud loss and formation damage. The authors will detail the operational procedures and the field applications of this drilling fluid, with particular emphasis on the lessons learned in the Lake Maracaibo implementation of the system.

fax 01-972-952-9435. AbstractDrilling depleted reservoirs is fraught with a host of technical and economic problems that often make it unprofitable to further develop some mature fields. Most of the problems center around uncontrollable losses and differential sticking. Frequently, less expensive drilling fluids will be used in a particular interval, even though it may have the propensity to damage the formation. The reasoning holds that such fluids will offset the high costs of losing more expensive muds to the formation. If operators turn to underbalanced drilling as an alternative, the extra time and equipment required for a safe operation can seriously degrade project economics in some applications.A specialized invasion-control drilling fluid has been developed to drill reservoirs prone to lost circulation. This fluid combines certain surfactants and polymers to create a system of micro-bubbles or aphrons that are encapsulated in a uniquely viscosified system.Aphron based systems are engineered drilling fluids that aid in well construction by controlling losses in depleted, high-permeability sands while stabilizing pressured shales or other formations. One of the more attractive features of an aphron-based system is that it does not require any of the extra equipment used in air or foam drilling. There are no compressors, high-pressure hoses or connections to add costs and safety concerns. The system uses conventional fluidmixing equipment to form tough, flexible micro-bubbles.This paper describes the development and application of the specialized micro-bubbles-based drilling fluid for controlling downhole mud loss in a depleted reservoir in the North Sea. The key issues of this project were excessive overbalance drilling conditions (> 5,000 psi) leading to the risk of highly expensive lost circulation and open perforations in the upper producer, requiring temporary sealing during drilling. The well was successfully drilled to TD without any drilling fluid losses. The authors will detail the laboratory methods used to generate appropriate formulations, the operational procedures, and field application.

Even in routine applications, the safety, economic and technical problems associated with lost circulation can severely impact drilling operations. The negative consequences are magnified greatly in the deepwater environment. Dramatic reduction in penetration rate and the downtime spent regaining circulation can further escalate already high operating costs. More importantly, the well control issues surrounding lost circulation pose critical safety concerns. In many of these wells, merely identifying the thief zone or zones is a major technical challenge. Furthermore, once identified, the vugs and fractures in the loss zones are at times too large to be bridged with conventional lost circulation material. This paper describes the development of a uniquely engineered lost circulation pill that when used in tandem with new real-time geomechanical analysis methods and pill formulation software, cured severe losses in the deepwater Gulf of Mexico. The authors will describe the development and laboratory modeling used in producing the specialized and chemically activated cross-linked pills, engineered to stop whole drilling fluid losses. As detailed in the paper, the pill proved to be far superior to conventional lost circulation material. Further, the paper will discuss its application in a well drilled in more than 2,800 ft of water in the Gulf of Mexico, which had encountered losses of up to 2,000 bbl of synthetic-base drilling fluid. The 100-bbl pill was formulated using a specialized software package. This was used in combination with a new process for analyzing the location, and extension pressure of the drilling induced hydraulic fracture using resistivity and annular pressure measurements. Once the pill was placed across the fracture zones, squeezed and later drilled out, normal drilling resumed with pre-fracture parameters resulting in penetration rates in excess of 50 ft/hr with no further losses. In addition to discussing the development and application, the authors will outline the lessons learned on the deepwater project featuring pre-planning issues geared toward ensuring circulation is maintained throughout the wellbore. Introduction The challenge of controlling fluid losses when drilling with synthetic-based drilling fluid systems in deepwater has been well documented1,2,3. The narrow operating window between pore pressure and fracture gradient often results in major fluid losses when drilling, running casing, and cementing. While drilling with synthetic-based mud delivers superior performance, the cost implications of massive losses often have forced operators to solve the problem rather than living with the consequences1. A technical and economic challenge associated with synthetic-based fluids is the fact losses are more difficult to cure, because of the tendency of fractures not to heal once closed. Consequently, the low fracture re-opening pressures allow for continual losses. Many products and techniques have been used in attempts to restore circulation while drilling. These include fibrous, flaky and granular materials2, as well as techniques such as gunk and reverse gunk squeezes, high fluid-loss squeezes and cement squeezes. More recently, a chemically activated cross-linked pill (CACP) has been developed and applied on challenging deepwater wells to cure and limit losses when drilling with synthetic-based fluids.

No abstract

Drilling depleted reservoirs is fraught with a host of technical and economic problems that often make it unprofitable to further develop some mature fields. Most of the problems center around uncontrollable losses and differential sticking. Frequently, less expensive drilling fluids will be used in a particular interval, even though it may have the propensity to damage the formation. The reasoning holds that such fluids will offset the high costs of losing more expensive muds to the formation. If operators turn to underbalanced drilling as an alternative, the extra time and equipment required for a safe operation can seriously degrade project economics in some applications. A specialized invasion-control drilling fluid has been developed to drill reservoirs prone to lost circulation. This fluid combines certain surfactants and polymers to create a system of micro-bubbles or aphrons that are encapsulated in a uniquely viscosified system. Aphron based systems are engineered drilling fluids that aid in well construction by controlling losses in depleted, high-permeability sands while stabilizing pressured shales or other formations. One of the more attractive features of an aphron-based system is that it does not require any of the extra equipment used in air or foam drilling. There are no compressors, high-pressure hoses or connections to add costs and safety concerns. The system uses conventional fluid-mixing equipment to form tough, flexible micro-bubbles. This paper describes the development and application of the specialized micro-bubbles-based drilling fluid for controlling downhole mud loss in a depleted reservoir in the North Sea. The key issues of this project were excessive overbalance drilling conditions (> 5,000 psi) leading to the risk of highly expensive lost circulation and open perforations in the upper producer, requiring temporary sealing during drilling. The well was successfully drilled to TD without any drilling fluid losses. The authors will detail the laboratory methods used to generate appropriate formulations, the operational procedures, and field application. Introduction The drilling problems associated with the depleted reservoirs intrinsic to many of the mature fields throughout the world often make further development uneconomical. The water-wet sands that typify many of these zones propagate seepage losses and differential sticking, both of which are extremely expensive to correct. Uncontrollable drilling fluid losses frequently are unavoidable in the often large fractures characteristic of these formations. Furthermore, pressured shales are often found interbedded with depleted sands, thus requiring stabilization of multiple pressured sequences with a single drilling fluid. Drilling such zones safely and inexpensively is very difficult with conventional rig equipment. Such problems have led some operators to forgo continued development of these promising, yet problematic, reservoirs.1 Excessive overbalance pressure generated when using conventional drilling fluids is thought to be the primary cause of lost circulation and differential sticking when drilling these wells. The equipment required to manage aerated muds or drill underbalanced is often prohibitively expensive, and meeting safety requirements can be an exhaustive effort. Furthermore, these techniques may fail to provide the hydrostatic pressure necessary to safely stabilize normally pressured formations above the reservoir. Recently, a new drilling fluid technology based on aphrons - uniquely structured micro-bubbles — was employed to successfully drill a depleted reservoir in the North Sea. The use of aphron-based drilling fluids has proven to be a successful and cost-effective alternative to drilling underbalanced. Description of Aphron Structure An aphron comprises two fundamental elements2:A core that is commonly, but not always, spherical. Typically, the core is liquid or gaseous.A thin, aqueous, protective shell with an outer hydrophobic covering. The aqueous shell contains surfactant molecules positioned so that they produce an effective barrier against coalescence with adjacent aphrons.