Recovery improved in a brownfield heavy oil well, using inflow control technology

As oil reservoirs age, the optimization of oil recovery becomes essential, if oil production targets are to be met. This is not least in heavy oil fields, where the challenge is greater, due to the lower reservoir energy and requirement for high reservoir contact.

Use of AICDs in heavy oil fields to control water can increase oil production.

One of the most important aspects of heavy oil is its viscosity, which can impact reservoir recovery and productivity directly. Although there is no direct relationship between density and viscosity, a reduction in API gravity scale index for heavy oil is generally accompanied by increased viscosity. Heavy oil usually occurs in shallower formations, in marginal geological basins formed by non-consolidated sand. Reservoirs tend to have lower pressures and temperatures in comparison to light oil reservoirs. This generally results in lower recovery factors. Although this characteristic points to more complex production processes, factors such as high permeability could make the recovery process easier.

Depending on the capillary pressure; gravitational and viscous forces; and the interaction between these elements during oil flow; oil is generally retained in the reservoir. The porous media may then be measured by the mobility ratio. The mobility of the fluids in porous media is defined by Darcy’s Law, which specifies a direct relationship between pressure, permeability and mobility, depending on viscosity and velocity. Due to an unfavorable mobility ratio for heavy oil flow compared to water, the primary recovery technique may leave as much as 70% of the petroleum in the reservoir.

APPLICATION OF AN ICD

To increase the recovery factor and production rates, supplementary recovery methods—generally termed improved oil recovery (IOR) techniques—deploy operational strategies and the supply of additional energy to the well to increase oil recovery. One technique used in horizontal heavy oil wells is the application of an inflow control device (ICD). This is an engineered nozzle or high-friction channel that is typically made from high-erosion resistance material, and installed in the base pipe to create pressure drop. It is used to control the inflow, from heel to toe, by applying higher pressure drop in the heel of the well to balance the inflow in the toe of the well and overcome the friction flow in the length of a horizontal completion, Fig. 1.

Fig. 1. Production, with and without an inflow control device in a horizontal well.

The use of passive ICDs in horizontal wells has been practiced widely in conventional wells, to achieve the balance flux and mitigate early breakthrough of unwanted water or gas in oil wells. The devices are, however, passive in nature, and once water or gas breaks through, the choking effect cannot be adjusted without intervention. Furthermore, the viscosity difference between heavy oil and water creates an unfavourable mobility ratio, which allows water to flow much faster through the reservoir and into the wellbore. This enables water breakthrough to happen faster, displacing oil output from producing zones.

Autonomous inflow control devices (AICDs) are designed to automatically react to the properties of the fluid flowing through them. An AICD restricts the flow of less-viscous fluids, such as water and gas, while allowing more-viscous fluids, such as heavy oil, to pass through with minimum pressure drop. When used in horizontal wells that have been compartmentalized using swell packers, AICDs restrict the flow of water in high-water-cut zones while allowing greater drawdown of the reservoir in high-oil-saturation zones, reducing water cut and improving oil recovery for the overall well.

Fig. 2. AICD construction.

Like ICDs, an AICD can be used in new wells to create a more balanced inflow profile along a horizontal section prior to water breakthrough. Once water breaks through in one or more zones, the AICDs restrict production from these compartments and favor production from low-water-cut zones. AICDs also can be used in existing wells, where water breakthrough has occurred already through deployment as a retrofit string, reducing water cut to extend economic well life and improving sweep efficiency. 

There are different types of AICDs, but most commonly used is the Flosure AICD, which comprises three components; valve body, nozzle and disk, Fig. 2. The FloSure AICD is constructed, using erosion-resistant material, and is engineered to fit within standard ICD housings without protrusion into the completion bore. 

AICD DEVICE IN OPERATION

AICD functions are based on Bernoulli’s Principle which, by neglecting elevation and compressible effect, can be expressed as:

P₁ + 1/2ρV₁² =  P₂ + 1/2ρV₂² + ∆P_{friction loss}              (1)

P₁ = Static pressure

1/2ρV₁² = Dynamic pressure

∆P = Friction pressure loss

The equation states that the sum of the static pressure, the dynamic pressure and the frictional pressure losses along a streamline is constant. In Figs. 3a and 3b, the streamline or flow path through the device is marked by arrows.

The AICD restricts the flowrate of low-viscosity fluids by increasing flow resistance. When gas or water flows through the valve, the pressure at the flowing side of the disk will be lower, due to the high fluid velocity. The total force acting on the disk will move the disc toward the inlet, and reduce the flow area and thus the flow, as shown in Fig. 3b. When more-viscous fluids flow through the valve, the friction loss increases and the pressure recovery of the dynamic pressure decreases. The pressure on the rear side of the disk will decrease, resulting in lower force acting on the disc toward the inlet, as shown in Fig. 3a. Thus, the disk moves away from the inlet and the flow area and the flow increases. Notably, the AICD cannot shut off production, but can only manage the rate as a function of the fluid properties.

Fig. 3a. AICD flow path and disc position with oil. Fig. 3b. AICD flow path and disc position with water.

APPLICATIONS

AICDs have been installed in a range of applications and reservoir types that can be described as horizontal oil producing wells with unwanted gas or water production. This includes sandstone; carbonate; heavy oil; open-hole and cased-hole, both as a retrofit solution and as a primary completion. The technology is applicable to both high-value subsea wells producing thousands of barrels a day, and low-yielding land wells producing just tens of barrels per day.

Fig. 4. Autonomous ICD flow path.

In sandstone reservoirs, the AICD is typically assembled as part of the sand screen joint in the lower completion. For carbonate reservoirs, the AICD can be deployed as a stand-alone sub, with a debris filter assembled before the inlet of the valve. The flow path in either configuration is the same, as reservoir fluids enter the completion through the filter and flow along the annulus between the filter and base pipe, into the inflow control housing where the AICD is mounted. The fluids then flow through the AICD and into the production conduit, moving to the surface together with the production from the rest of the well, Fig. 4.

AICD SUITABILITY TO HEAVY OIL APPLICATIONS

Full-scale tests on realistic heavy oil conditions have been completed. Single-phase and multi-phase experiments with 27-cp crude oil and water were performed to define the characteristics of the AICD. The characteristics are described by the differential pressure across the AICD versus flowrate through the device.

The AICD mathematical function described by the differential pressure across the AICD is expressed by the function f(ρ,µ):

                (2)

a_AICD, x and y are user input strength parameters based on nozzle size, q is the local volumetric mixture flowrate and the user inpouts ρ_cal and µ_cal are calibration density and viscosity, respectively. The ρ_mix and µ_mix are flowing mixture density and viscosity at downhole conditions:

                    (3)

                   (4)

Items a, b, c, d, e, and f  have been implemented recently to the mixture equations, to aid better descriptions of the mixture properties at multi-phase conditions.

Fig. 5. Multi-phase production test results, with AICD as a function of volume flowrate and pressure drop.

 
Figure 5 shows two-phase oil/water tests performed with water cut at 25%, 65%, 80% and 92%. The pressure drop, as a function of total volume flowrate, is plotted together with the single-phase oil and water curves for reference. With this degree of viscosity contrast, water will travel faster at a similar pressure gradient compared to oil. The AICD imposes a much-higher pressure drop on water and leads to a reduction in water flow.

The mixed oil and water generates a mixture viscosity, depending on the fraction of each fluid. The same trends were observed with increasing water cut. As the water cut increases, the mixture viscosity will be reduced, and the velocity of the mixed fluid flow increases through the valve, resulting in a gain in the pressure drop. Based on the results, the oil-water volumetric flow ratios for 27-cp oil can be calculated. At a 15-bar pressure drop, oil/water ratios are about six times. This means that when the zones have water breakthrough, the AICD will choke the flowrate by almost 80%.

It is observed during the test at 25% water cut, that the AICD is not effectively choking the fluid mixture, due to a small change in mixture viscosity. Thus, oil is still in a continuous flowing phase. When the water cut increases to 65%, the AICD starts to show increased choking. As the water cut increases to between 80% and 92%, the choking behavior strengthens, resulting in reduced viscosity. A viscosity contrast of approximately 3 cp is required for the AICD to differentiate between water and oil. It follows that as the viscosity contrast between oil and water increases, the effectiveness of the AICD increases, making this technology particularly suited to heavy oil applications.

ANALYSIS AND SIMULATION TECHNOLOGIES

Multiple commercial nodal analysis tools and dynamic reservoir simulators are equipped with AICD function in the inflow control device option. A static reservoir simulator can optimize the AICD size and number of AICDs per joint. Dynamic simulators are required to quantify the production benefits of the AICD over field life. Notably, all new wells are simulated with a static and dynamic simulator, to design the AICD size and number of AICDs per joint.

For example, in wells without inflow control, the water may be drawn into the wellbore from the down-dip oil-water contact through high-permeability channels, reducing effective drainage of the oil up-dip. The AICD will improve the water sweep by balancing the inflow from high- and low-permeability sections, and creating additional pressure drop at high-water-cut zones. Furthermore, the AICD will allow a low-mobility, viscous oil to be produced and recover the oil up-dip.

Enhanced well performance can be achieved with AICD-completed wells, providing that acceptable production rates can be achieved through the AICDs, throughout the life of the well. As viscosity contrast between the oil and the unwanted fluid (gas/water) is present at well conditions, some heterogeneities or non-uniformity in water production is present along the wellbore, and there is the ability to achieve adequate compartmentalization of the wellbore annulus.

Candidate selection starts with an evaluation of the increased or accelerated oil production. Once this is determined, well operability factors can be considered during the detailed well planning phase. A main factor affecting the results of this process is the reservoir fluids.

The AICD requires a viscosity contrast to provide additional increased or accelerated oil production. This will be the case for water control in oil fields. AICD has the potential for controlling water breakthrough/water production in heavy oil applications. Comparison of the flow characteristics of AICDs versus passive ICDs for the reservoir fluid is the first screening stage for a new application. This is performed using regression analysis to define the AICD characteristics, based on the viscosity and density of oil and water at reservoir condition.

Fig. 6. An AICD performance comparison with an ICD.

Figure 6 shows an example of test conditions with an oil viscosity of 27 cp. The single-phase oil flowrates for AICDs and ICDs are matched for a given pressure drop, and the differences in flow characteristics are examined.

COMPLETION DESIGN

Effective compartmentalization is critical in AICD completions, to allow different choking between compartments, enabling more contribution from sections with the highest oil fractions. Generally, the more compartments that can be created, the more effective AICD performance will be. Limiting unwanted water production into smaller wellbore lengths allows greater contribution from zones with higher oil saturations.

Limitations on the number of compartments may be imposed by well operability factors, such as hole condition, weight limitations or existing completions. The location of compartments can be determined by changes in reservoir factors, such as natural barriers, fractures, permeability or saturation contrasts. In longer bores, locations may be simply spaced out along the completion.

Sensitivity studies using nodal analysis tools are required to optimize the quantity and location of zonal isolation devices. This analysis is typically reviewed, once actual well data become available, and changes can be made at the rig site. Zonal isolation is typically achieved using swellable rubber, mounted onto sleeves that can be slipped over the completion tubing and secured in place, providing a flexible solution to compartmentalization.

Fig. 7. A retrofit AICD completion in an existing, stand-alone screen, with the production flow paths.

For retrofit applications in inner strings consisting of installing AICD subs, swellable packers are installed within the existing wellbore. In this case, compartmentalization is driven by the existing wellbore, whether that be stand-alone screens or gravel-packed completions, along with packers for zonal isolation (Fig. 7), or with cased and perforated wells.

The quantity and sizing of AICDs (driven by changes to the inlet port) will depend on the overall flowrate, formation productivity and the well length. Each AICD has a flow capacity as a function of its inlet port diameter. Total capacity of the completion must be equal to, or greater than, the target production rate for a given flowing condition. The initial and maximum oil and liquid production targets will be simulated to determine the quantity and size of AICDs required to ensure that maximum well deliverability is achieved.

The evolution of water cut, over time, is critical for AICD completion design to maximize oil production. Value is derived from the application of AICDs, if water breakthrough can be delayed, and if water saturation development is non-uniform along the wellbore. During early production, prior to water breakthrough, AICDs can optimize drainage and reduce the likelihood of water coning by ensuring that inflow between the zones is balanced. This provides a window to accelerate early oil production, and then maintain output from high-oil-saturation zones, when water begins to break through other zones, until water saturation increases uniformly along the entire wellbore. Once water saturation is uniform along the wellbore, the AICDs no longer improve production performance.

The AICD also acts as a check valve, preventing flow from the production conduit to the formation (injection direction). During deployment, this allows circulation through the completion without deploying a wash pipe, and allows a hydraulic packer to be set. In later life, where chemical treatments are prescribed to treat scale, paraffin or asphaltene problems, requiring injection into the wellbore annulus and/or the formation, a bypass valve can be installed, adjacent to the AICD to permit injection. The bypass valve can be installed as part of a screen assembly (with the AICD) in the lower completion, or run as a separate sub.

REDUCING WATER CUT: A CASE STUDY

AICDs have been used in brownfield wells across Europe, the Middle East, China and North America as a retrofit solution after water cut increases, most commonly when water cut has reached up to 96%.

Fig. 8. One of the first AICD retrofit installations offshore China boosted oil output significantly.

In one of the first AICD retrofit installations during December 2014, in a heavy oil environment offshore China, designed to control water cut, it also showed a significant increase in oil production, Fig. 8. The length of the well is 600 m horizontal, and it was completed initially with 5.5-in. screen, with gravel pack in an 8.5-in. open hole. Retrofit AICDs have been installed on 4-in. pipe joints and deployed inside existing 5.5-in. screen. The well was shut in previously, due to the water cut exceeding 96%. Following installation of the AICD completion, a reduction in water cut to 93.6% was observed. The water cut reduction enabled a resultant increase in oil production from 43 m³/d to 55m³/d, or 28%. Based on the positive results of the initial well, there have been many more wells within the field completed with AICDs, as a retrofit solution or primary completion for new wells.

One of the key challenges in successfully implementing AICD in this field, as with many late-field life applications, was the limited availability of well performance data, production logs and dynamic reservoir models. To demonstrate the potential value of AICD technology, a series of hypothetical scenarios was created, and a statistical approach adopted.

Where water saturation is at 96% across the producing section, the AICDs cannot add value. But where variation in water cut increases value, it can be demonstrated, provided the additional pressure drop resulting from the AICD does not limit well production, Fig. 8. In this case, progressive cavity pumps were used to drive production and, therefore, low reservoir energy was not a limiting factor. Similarly, a statistical approach was adopted to evaluate the success of AICD applications, with a range in increased oil production of zero to 165%, but with an average of 44% immediately following installation.

Using AICDs in heavy oil fields can help reduce water cut. The viscosity difference between heavy oil and water provides a favorable mobility ratio well-suited to this technology and has been shown to increase oil production. The trial well was flowing with an initial water cut of 96%. After installation of AICD, the well produces with a 93% water cut. It is envisaged that the water is coming from wet sand in the heel of the well, and the AICD is choking the high-water zone area. The water cut reduction enabled a resultant increase in oil production from 43m³/d to 55m³/d, or 28%. As the water is restricted upon breakthrough, the overall recovery of the well is improved, when compared to operations using conventional methods and passive ICDs.

Use of AICDs requires a thorough understanding of the technology, well performance and downhole fluid properties that will impact the design and determine ultimate recovery. The reservoir model will have at least some uncertainty that needs to be addressed, using sensitivity analysis to ensure that the AICD performs in the well, in all scenarios.