Cementing the 20" surface casing in offshore Angola is associated with low fracture gradients and shallow water flows. These particular challenges require the use of lightweight slurry with both bead materials, and tight fluid loss control which was the initial approach for the section mentioned above. Due to high slurry volumes, rig site bulk logistics deployment, and budget, Foam cement was proposed. The Paper describes the design methodology and implementation of an automated foam cement system with real time monitoring. Based on the challenges presented above, Foam cement slurry has been the choice for some operators in Angola. Using a constant N2 injection rate with a foaming agent and a stabilizer that are injected into the slurry prior to it reaching the foaming "T" with variable choke bean size where N2 is injected into the slurry prior to pump downhole. Specific developed software’s, and Local training classes with in class and field training have proven to be very efficient allowing to have flawless execution pre and post job comparison and data evaluation. The foamset cement system has shown to be very reliable and efficient on covering shallow surface areas likely to preserve fresh water aquifers. A standard slurry design was developed; thickening time adjusted depending on well conditions. The slurry was stable both at surface and downhole conditions with foam quality varying from 20% to 35%, providing required compressive strength. Since the implementation in Angola, 26 jobs have been performed successfully. This slurry can undergo a wide range of downhole pressures and temperatures variations without deforming the cement in place and or compromising well integrity. This paper will share experience and success acquired in offshore Angola with best practices used in laboratory testing, both operations and engineering planning, execution, and post job data results.

Maintaining the density hierarchy for wellbore fluids has been a routine while achieving a proper rheological hierarchy for mud, spacer and cement could have been compromised due to tedious testing and sometimes limitations in the field. Establishing appropriate rheological and friction pressure hierarchy prevent fluids (mud-spacer-cement slurry) intermixing especially in deviated and horizontal wells. The objective of this paper is to present a spacer rheological properties model along with a new micro-emulsion spacer formulation which improves well integrity. This water-based spacer system, with densities ranging from 8.5 to 16 ppg, was modeled to temperatures up to 325°F and provided proper suspension properties, confirming stability at bottomhole circulating elevated temperatures. In addition, ccompatibility of this spacer package with various synthetic based muds, oil based muds and cement slurries, designed for Gulf of Mexico, the US land, North Sea and the Middle East, plays a significant role in achieving great displacement efficiency, wellbore clean up, long term effective zonal isolation and sustainable hydrocarbon production. It is not always possible to accomplish the turbulent flow. Therefore, a rheological model was developed to accomplish the ideal viscosity hierarchy by optimizing the spacer formulation design. Optimum rheological hierarchy occurs where the viscosity profile of a spacer system is higher than the viscosity profile of drilling fluid and lower than the cement slurry. Model's predictions have been validated by one atmospheric and two industry-known HPHT rheometers. The model predictions show that the rheological profiles of the spacer fluid, for all the main standard shear rates, are between the mud and cement profiles. Data obtained from field case histories show the improvements and added values such as ideal fluid compatibility, better displacement efficiency, friction pressure hierarchy and effective zonal isolation,

Insufficient wellbore cleaning prior the cementing job is considered to be the biggest single factor leading to poor zonal isolation results. A mud-spacer-cement program with suitable fluid needs to be carefully engineered for the given wellbore conditions to improve cementing quality. We discuss optimum spacer design features which are critical for the successful cementation of deep deviated HPHT wells containing heavy oil based muds and review a simulated scenario. Advanced lab test methodologies beyond industry standards are utilized to model more accurately the given complex downhole conditions. A simulated >20,000 ft highly deviated wellbore was characterized by HPHT bottomhole conditions and the rheological performance of the cement spacer was critical to job success. The well needed a stable cement spacer that would not settle-out on the low side of the >14,000 ft horizontal section, which would potentially put the well at risk. The 16.17 ppg mud required an even higher-density spacer system to clean it effectively. But conventional high-density spacer systems only compound the settling challenge and the well's anticipated bottomhole temperature of 350°F was expected to compromise any additives that might stabilize the fluid systems. Therefore a lab study about spacer stability was performed using a HPHT rheometer and the dynamic settling test – an industry standard which was actually established for cement slurries but not for spacer fluids. We found that a conventional spacer failed at 350°F by showing a rapid decline in rheology to almost zero viscosity and severe settling. To overcome the settling issue, provide stability, and maintain a sufficiently high rheology profile at given 350°F, we re-designed the spacer by using a modified biopolymer which shows a delayed hydration and viscosification over time successfully counteracting the destructive thermal effects. The mud-spacer-cement fluid train was eventually optimized showing good fluid compatibility and maintaining within the narrow, 1.6 ppg margin that separated the pore pressure from the fracture gradient. The cementing job was designed using an advanced fluid displacement software, which predicted high mud removal efficiency under these challenging conditions. In order to enable proper mud displacement, the Friction Hierarchy—a key design factor that is often difficult to achieve under the extreme HPHT well conditions—was achieved with the new spacer concept.

The quality of zonal isolation and well integrity are two main objectives for a successful cementing job. These objectives require proper placement of cement in the annular depth interval of interest. Cement placement, in turn, is dependent on effective drilling mud removal. A spacer fluid is designed to aid displacement of the drilling fluid and to minimize cement contamination. It takes into consideration not only wettability tests and compatibility results between spacer/mud and spacer/cement, but also rheological properties to improve friction pressure hierarchy. The rheological properties vary depending on flow rate, polymer concentration, spacer density and temperature. Trial-and-error lab tests and re-designs are needed to establish a spacer to meet those requirements. To optimize the costly process, and to expedite the job design, it is necessary to propose a smart rheological hierarchy optimization methodology that can predict the spacer rheologies and polymer concentrations by leveraging information from known data. In this paper, a novel rheological hierarchy optimization methodology with artificial intelligence and inverse technique is introduced to improve spacer design. First, sufficient existing rheological property data (FANN readings) as a function of spacer density, temperature and polymer concentration are used to train the machine learning algorithm. Second, depending on the selected mud and cement, density of the spacer and the bottom-hole temperature, the required spacer properties are predicted by the trained machine learning algorithm for each polymer concentration. Third, given the wanted rheological properties, the best solution can be found by the inverse optimization algorithm with the least- square error in the range of polymer concentration. The proposed methodology is applied to a new class of spacer system, which is a premium, water-based spacer, designed to effectively displace the drilling fluid in the annulus. Hundreds of rheological properties are recorded with various combinations of density, temperature and polymer concentration. The data are used in training the nonlinear machine learning algorithm. In real cement-job design, with the required FANN readings of the spacer and FANN readings of mud and cement, the algorithm finds an optimized solution. The FANN readings predicted by the machine learning algorithm are compared in the inverse search step. The calculated polymer concentration and FANN readings are confirmed by laboratory tests with minor variations.

Achieving the turbulent flow regime might not be always possible in the field therefore cement engineers follow the ELF (equivalent laminar flow) rules used for efficient cement placement. These rules are: 1) Density hierarchy: The displacing fluid should be at least 10-15% heavier than the fluid it is displacing. This percentage is dictated by the pore pressure-fracture pressure window. 2) Friction pressure hierarchy: The friction pressure of the displacing fluid should be at least 20% higher than the fluid it is displacing. The advanced fluid displacement simulator fluid friction charts should be used to determine this. 3) Minimum pressure gradient: Each displacing fluid should be able to break the gel strength of the fluid it is directly displacing to prevent leaving any gelled, undisplaced layers in the annulus. 4) Velocity profile: The velocity of the displacing fluid in the wider side should not exceed the velocity of the fluid being displaced in narrow side of the same annulus in order to effectively remove mud from narrow side of the annulus. Friction pressure hierarchy modeling helps minimizing fluid intermixing in the annulus. Rheological hierarchy optimization is in fact a critical step towards more accurately designing the spacer fluids for enhanced zonal isolation. Besides the required laboratory testing for mud, spacer and cement, designing cement jobs with the minimum fluid intermixing and viscous fingering will improve zonal isolation. The objective of this paper is to perform a sensitivity analysis utilizing a micro-emulsion spacer rheological properties model and a 3-D visualization computational fluid dynamics (CFD) module to investigate the optimum rheological profile for a weighted spacer fluid to maintain the friction pressure hierarchy and enhance zonal isolation. Based on the simulation results the optimum scenario was selected. This engineering analysis positively yielded in a good cement job confirmed by CBL/VDL.