Proven Synergistic Approach in Well Cementing Across Thief Zones: East Venezuela

Abstract

Drilling operations for several operators in East Venezuela are increasing in complexity each year. Many challenges are encountered while drilling across weak formations with considerable variability in lithology and pore pressure. A narrow drilling window between formation pore and fracture pressures has become one of the main concerns for the operator. Primary cement placement faces similar challenges when placed across weak formations. New producing zones are currently being targeted in deeper sandstone formations and a variety of loss circulation issues are encountered. For example, in the Tacata field, lost circulation is occurring across a series of depleted zones located above the producing zone. In the Orucual field, losses were observed immediately below the producing zone. From the wells observed in this study, losses were observed most frequently under slim-hole conditions and low fracture pressures. In these wells, combinations of demanding well and formation parameters have made it more difficult to achieve successful cement placement. Minimizing losses during cement placement, achieving predicted top of cement (TOC) and reliable zonal isolation are current challenges in East Venezuelan fields. The need to improve conventional lost circulation approaches during well drilling and cementing was determined by service company and operator field experience and dynamic cement placement simulations. A high performance light weight (HPLW) slurry was recommended along with an engineered fiber material (EFM) to avoid losses by reducing the hydrostatic pressure in the annulus and artificially increasing the fracture pressure of the thief zones during placement. From several case histories, the synergy between HPLW and EFM approaches resulted in improved zonal isolation, less reservoir damage and cost savings by avoiding remedial cementing and non productive time (NPT). Introduction The main Venezuelan oil fields are located in the northeast part of the country. Monagas state has the majority of the principal oilfields such as Oficina Major Area, Quiamare, JusepÍn, El Furrial, Orocual, Boquerón, Tacata and Quiriquire. Some fields (e.g. Tacata) have more mature zones in shallow to intermediate depths that are reaching depletion and deeper producing sandstones are being targeted. Other fields (e.g. Orucual) possess thief zones immediately below the producing zone. Loss circulation is occurring across these depleted zones due to the narrow well security window. The well security window is defined as the operating range between the pore pressure of the production zone and fracture pressure of the depleted zone. Challenges and Problem Analysis Operating outside the well security window incurs additional costs and unnecessary risks to personnel and equipment during well construction. Due to lost circulation in these fields, average NPT and mud losses exceed 48 hrs and 2,000 bbl, respectively, during the 17.5 in drilling phase for each well. These problems must be addressed by careful planning/design and risk analysis to reduce uncertainty to an acceptable level. For example, when losses are anticipated during drilling operations, careful drilling program design must consider several parameters, such as hole geometry, mud type/density, rate of penetration, surge and swab pressures, casing point selection, etc. In narrow well security windows, casing point selection and cementing programs are also more complex with the added limitations of slim-hole conditions and increased friction pressures during cement placement. In order to reduce friction pressure, the operator may decide to reduce placement rates and increase the job duration. Reduced placement rates may contribute to increased risk for insufficient mud removal, incomplete zonal isolation and annular gas migration, which may require remedial squeeze cement methods to correct the problem. For example, several wells were tested under leakoff conditions following completion of intermediate and production strings using convention cement placement methods. General field observations by service company and operator personnel noted an increased leak-off failure rate. In strings that failed the leak-off test, at least one remedial shoe squeeze job was required to repair the problem. This added substantial cost to the operator in remedial cementing and NPT.