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.

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In a deepwater environment, any remedial operation has a high impact on the overall costs during the life of the wells. The zonal isolation can be compromised due to the exposure of the well's main components (casing and cement) to the changes in the stress conditions. The changes in wellbore conditions can occur during the drilling, production, intervention, and decommissioning stages. Typically, conditions such as fluid pathway and high formation pressure are sufficient to lose zonal isolation. The fluid pathway can be a fissure, an induced crack in the cement sheath, a mud channel, a micro-microannulus, or changes in the cement matrix permeability. As a result of the oil industry technology developments, progresses, the advanced stress-modelling software and the availability of cement and rock properties property data have enabled to an improved understanding of the cement behavior under stress. Prevention of the loss of the hydraulic isolation provided by the primary cementing in the annulus can be assessed by predicting the mechanical failure of the cement sheath. Formation geo-mechanics is one of the main factors that help in designing a robust cement system for changing stresses. Furthermore, the consequence result of the cement sheath failure can be mitigated by the placement of placing a self-healing cement (SHC) system to maintain long-term zonal isolation. An interdisciplinary approach can be used to determine the following: Understand the impact of the well plan, and fluid densities on well integrity, in addition to cementing best practices.Characterize typical deepwater field formations, and establish limits for geo-mechanical values of each layer.Identify critical factors and focus on the pay zones.Understand potential issues and communication between the pay zones and the aquifers that are already previously confirmed.Determine risk of zonal communication assessment, mitigation, and prevention measurement implementations Once the formation data is validated by the operator, the life cycle of the well is simulated and the risk of zonal isolation can be evaluated. The results of this assessment can help the operator choose between to take the approach of mitigation, prevention, or a combination of both. The objective is to place a robust cement sheath with advanced mechanical performance in the pay zones that can resist the failures due to changing stresses during the well testing and production. This paper uses presents examples from a deepwater development field to show how cement systems with advanced mechanical properties counter the critical stresses during the lifecycle of a well and maintain zonal isolation.

In a Deepwater well off the Brazilian coast which presented a complex architecture with multiple drilling casings and liners, losses were expected during cement placement across a carbonate formation. This paper describes the use of a new real time monitoring and evaluation tool which takes the data acquired during the cement placement, then processes and simulates in real time to provide important job parameters such as estimation of fluid interface positions inside the casing and annular space, pressure match chart, density quality assurance and quality control (QA/QC), ECD and dynamic well security, among others. This manuscript present two cases history where the operator and the service company work together to define a decision tree for the possible contingencies related to unwanted TOC based on mud losses or unplanned cement placement. Later during the operation the new tool combines the design data with the cement unit and rig acquisition data to compare the job measured surface pressure, density, flowrate and volume with predicted data from simulations. Finally based on the information of real time estimation of the TOC outside the pipe and annulus space observed during the job execution a contingency from a decision tree is taken. The cementing service company provided real-time monitoring and evaluation tool that allowed the operator to identify the estimated TOC at the end of placement. With this information, the client was able to avoid the top of liner squeeze and save 2-3 days rig time Later a cement bond log showed that top of cement was found between the liner lap confirming the barrier element. In another case it was prevented leaving unplanned cement inside the casing with the analysis of the job and simulated pressure match trends at the end of the displacement and eliminated unexpected flat times for additional drill out time. Real-time monitoring and evaluation is a tool that can be deployed not only in Deepwater wells in Brazil, but in any section of wells being drilled around the world on land, on the shelf or in Deepwater, where the operator wants to visualize ether the deviation of job execution from job design parameters or a prompt estimation of top of cement as a first level of detection for the well barrier placement just after bumping the plug. In addition having the real time dynamic ECD will also aid in avoiding any potential well control situations (including lost circulation) during the cement operations at any time during this critical activity