Wellbore hydraulics R&D supports evolution in Brazil’s deepwater, presalt

Petrobras technology implementations pay off through field results in tough drilling scenarios

By A.L. Martins, A.F.L. Aragão, P.E. Aranha, M.G Folsta and A.T.A. Waldmann, Petrobras

Approximately 80% of Brazilian oil production comes from offshore fields. Campos Basin is the major producing area while Espirito Santo and Santos represent relevant reserves for the future. In most offshore scenarios, Petrobras faces deepwater (greater than 800 meters) and ultra-deepwater (greater than 2,000 meters), as well as reservoir-related issues such as salt drilling, nonconsolidated sands, heavy oil reservoirs and heterogeneous carbonates.This article presents a brief history of offshore well construction in Brazil, along with a discussion of wellbore hydraulic and the main R&D efforts focusing on hydraulics that the company has undertaken in association with a number of Brazilian universities.

Water depths in Brazil range from 500 meters to 3,000 meters while reservoir depths are typically 3,000 meters to 3,500 meters. Pre-salt cluster reservoirs are deeper (5,000 meters to 6,000 meters).

Development wells for the Campos Basin are typically directional or horizontal. Horizontal sections ranging from 500 meters to 700 meters are common in deepwater developments, and these long horizontal sections pose the first challenge for well trajectory. Particularly in heavy oil reservoirs, to maximize the reservoir area and guarantee an economical flow rate, a big challenge is to assure sand control in horizontal sections that are longer than 1,200 meters.

Another big challenge is drilling deepwater extended-reach wells. This may be a good option for dry completion projects, or even to consider shallow-water vessels to drill deepwater reservoirs. A ratio of lateral departure and vertical depth greater than two is considered to be critical and challenging.

Major issues in hydraulics include drilling fluid substitution and completion/ sand control, described below.

Monitoring Pressures during Drilling Operations

The main goals of a good hydraulic project are to obtain proper hole cleaning and maintain pressures inside the operational window. Another important issue is to make good use of real-time monitoring of downhole pressures. The industry has invested significantly in sensors and data transmission, but real-time interpretation is still far from the desired levels.

In some conditions, drilling hydraulics may limit the feasibility of the construction process. Dynamic pressures should be maintained inside the operational window defined by the pore, collapse and fracture pressures, guaranteeing that no influxes, losses or rock instability issues occur while drilling. Moreover, minimum flow rates are required to ensure that adequate drill cuttings transport occurs.

Downhole pressures are generated by two origins: hydrostatic forces and friction losses. Hydrostatic depends on fluid density and vertical depth while friction losses depend on fluid density and rheology, flow rate, flow geometry and flow path.

The hydrostatic term depends on the solids concentration and solid density-fluid mixture in the annulus. The solids concentration, on the other hand, depends on the cuttings transport process, affected by fluid rheology, flow rate and wellbore geometry. In critical scenarios, where the operational window is narrow, annular friction losses start to play an important role in downhole pressures. This is especially critical in deepwater reservoirs, deep reservoirs and fields that require long horizontal wells.

Minimizing downhole dynamic pressures depend on keeping good hole-cleaning conditions by defining an adequate rheological profile for the fluid and using proper drillstring geometry. Normally, in overbalanced drilling operations, the fluid density is designed in a way that a comfortable overbalance exists between the wellbore hydrostatic pressure and the formation pore pressure.

Fluid Displacement

There are two specific situations where minimizing contamination while displacing fluids is mandatory. First, when displacing a synthetic drilling fluid by water, there is a tendency for contamination in the riser due to the low annular velocities. Excessive amounts of contaminated fluids may generate environmental concerns and additional transport costs.

Another important situation is during the substitution of the drill-in fluid by a solids-free completion fluid in open-hole horizontal sections. In this case, any drill-in fluid left in the well may have detrimental effects on well productivity. Primary and plug cementing operations also require optimal displacement to guarantee proper zonal isolation.

Open-Hole Gravel Pack

This is the primary sand-control strategy adopted by Petrobras for offshore horizontal wells. Open-hole gravel packs consist of filling the open space between the wellbore walls and the production screens with sized gravel, generating a high-permeability pack, which would be able to allow oil production but restrict sand. The conventional operation is based on the conventional alpha-beta wave placement and is associated with large pressure drops during its final steps. During beta wave placement, the flow is diverted to the narrow annulus formed by the screen and the washpipe. In long horizontal wells, downhole pressures may exceed operational limits.

In the early 1980s, the discovery of the Marlim and Albacora fields in the Campos Basin led to Petrobras gradually changing its profile into a deepwater company. To provide technical support for challenges encountered on these fields, the company started a corporate R&D program in 1986 called PROCAP. The deepwater technological program focused on areas such as reservoir engineering, well construction, flow assurance, subsurface equipment, surface facilities and risers, aiming at water depths up to 1,000 meters (3,300 ft).

Well construction challenges were encountered with the drilling of inclined wells in these scenarios. Wellbore stability and cuttings transport started to play a major role, especially after stuck pipe events. During this period, Petrobras’ well technology R&D team built a comprehensive two-phase flow model to account for cuttings transport in horizontal and highly inclined wells.

In parallel, experimental work concerning friction losses and cementing design for inclined wells were also performed. Knowledge about hydraulics design tools were very limited at this time and had to be gradually gained with experience.

The Marlim field was developed in the ’90s mainly with open-hole completion horizontal injector and producer wells, with 400 meters to 600 meters of horizontal length. Technological development was required to guarantee sand control: After the failure of prepacked screen technologies, Petrobras pushed the service industry to support the implementation of open-hole gravel-pack technology for open-hole horizontals.

In 1991, a new corporate R&D program called PROCAP 2000 was started to support new discoveries in deeper waters (1,000 meters to 2,000 meters), including new phases Marlim, as well as Marlim Sul and Roncador. This program lasted until 2000 and hosted several technology projects focused on extended-reach wells and complex-trajectory wells – both situations where hydraulics played a fundamental role.

Cuttings transport modeling evolved for transient approaches and coupling with shale stability analysis, flow loop experiments and the search for field measurements of cuttings removal. During this period, successive versions of the in-house hydraulics software were delivered and widely used within the company. Feedback from field staff and continuous improvements gained from modeling and experimental work made this software a reference in the company even today.

An important achievement of this period was the construction of the extended-reach well 7-MLS-42H in 2001. The well, at the time a record in oil production, was drilled in 1,212-meter water depth to a depth of 5,211 meters – 2,896 meters vertical depth and 3,528 meters lateral departure.

In the past decade, as a result of the increase in exploratory drilling in ultra-deepwater, Petrobras has discovered new areas with heavy oil and, more recently, light oil in the pre-salt cluster.

PROCAP 3000 was launched in 2000 to support the ultra-deepwater scenarios for Marlim Sul and Roncador and other challenging exploratory prospects. Hydraulic topics included:

• Real-time decision systems based on downhole pressure data. The authors proposed a methodology and software to anticipate drilling problems based on PWD and other downhole data, mudlogging data and artificial intelligence techniques. The system has been in continuous improvement through tests run at rig sites and onshore decision support rooms.
• Managed pressure drilling (MPD) for floating platforms. The projects aimed to test and validate commercial systems in Brazilian offshore environments.
• Minimizing drilling fluid invasion into reservoir rocks. In these studies, the authors proposed a methodology to optimize drilling fluid composition for given reservoir properties. Rig-site testing, inverse log analysis, anisotropic and compressible flows were relevant topics in the study. Figure 1 highlights an invasion analysis for a deepwater exploratory well.

In October 2002, Petrobras launched PROPES, a program focusing on offshore heavy oil fields. It covered most upstream disciplines and included an interface with the downstream area. The objective was to develop or integrate existing technologies that may turn into reality the challenge of producing heavy oil discovered in the Campos and Santos Basins. Hydraulic topics included:

• Drilling and completion of long horizontal well sections in deepwater environments. The focus was on gravel packing, Petrobras’ preferred sand control strategy. Several strategies were developed to guarantee proper gravel-pumping within the operational window.
• Optimization of hydraulic parameters to drill heavy oil reservoir sections: Flow loop tests conducted with synthetic nonconsolidated sand cores saturated in heavy oil indicated that water-based muds and low Reynolds numbers minimize leach ing. CT scanning was used to monitor hole enlargement during the tests (Figure 2).
• Fluid substitution in deepwater long horizontals: Fluid contamination in the riser and open-hole horizontals led to technical and environmental concerns. Cementing quality was also heavily affected by poor fluid substitution. Simulation of transient displacement flows enabled the establishment of dedicated pumping and spacer pill design procedures for each specific scenario. Aranha et al (2011) describe useful procedures for cement plug displacement, still a risky task today in deepwater scenarios where free-fall effects cause flow rate fluctuations while pumping. Figure 3 illustrates simulation results for completion fluid displacing drill-in fluid in a horizontal section.

In 2007, Petrobras announced the discovery of huge oil accumulations below the salt layer in the southeastern shelf. The development of such fields imposed the need for new technological development in several E&P disciplines, but with special focus on reservoir geology and well construction. To address these issues, PROSAL was launched. Important well construction issues included drilling build-up sections through salt zones, optimizing ROP in hard-rock reservoirs and stimulating heterogeneous carbonates. Hydraulic concerns address the following aspects:

• Minimizing the interaction between the drilling fluid and salt zone, avoiding leaching and change in fluid properties. Flow loop experiments with real cores and CT scanning were conducted to optimize fluid composition and hydraulic parameters for different salts (Figure 4). Mechanistic models to predict leaching and define design parameters while drilling and cementing salt zones were developed. Typical outputs are highlighted in Figure 5.
• Dynamics of acid wormholing generation in carbonates, including CFD simulation and experiments.
• Minimizing losses while drilling rubble zones and naturally fractured reservoirs. Figure 6 illustrates a unique strategy coupling computational fluid dynamics (CFD) with discrete element methods (DEM) simulation for bridging fractures.

Fundamental Work

In order to achieve the technological results required in the previously described programs, a team effort was necessary. While R&D and E&P staff in Petrobras

was in charge of identifying priorities for future developments, foreseeing future trends, coordinating the projects and conducting internally strategic tasks, several other institutions played important roles in the process. Universities and research institutes accounted for fundamental and applied research, and service companies were the natural gateways for the introduction of new technologies. Whenever possible and convenient, partner operators shared costs, risks and benefits of the introduction of new technology.

Supported by Brazilian legislation, with R&D investments in Brazilian institutions with tax reductions, several projects have been developed by Brazilian academia in the past decade. Well construction projects were grouped into the Well Construction Technological Network – REDEP, with an annual budget of approximately US$20 million. Several universities and research institutes participated and conducted relevant wellbore hydraulics projects (Table 1).

This article presented information about the R&D and technological implementation efforts on hydraulics by Petrobras and its partners. Relevant field results included:

• Massive exploratory drilling and reservoir evaluation in subsalt/ultra-deepwater environments.
• Offshore extended-reach wells.
• More than 270 open-hole gravel packs in sections as long as 1,200 meters.
• 2,000-meter horizontal section drilled in shallow-water carbonate heavy oil reservoir.
• Unique managed pressure drilling offshore experiences.
• Open-hole gravel packing with synthetic low-viscosity fluids.

Future scenarios present additional challenges that need to be addressed. All technological limits will have to be pushed to economically develop newly discovered fields.

This article is based on SPE/IADC 140145, “Well Construction Hydraulics in Challenging Environments,” presented at the 2011 SPE/IADC Drilling Conference & Exhibition, 1-3 March, Amsterdam.