The challenge of maintaining deepwater wells

Technology changes for deepwater wells improve process, but they also add complexity.

03/29/2016


Figure 1: A shallow water well schematic. Courtesy EKTInteractive, Inc.In the last 25 years there has been a rapid advance into deep and ultra-deep depths facilitated by the industry's development of new technologies. Such production is primarily from subsea wells; that is, wells completed on the seafloor. Today, operators are producing from almost 10,000-foot depths in the Gulf of Mexico.

The technologies that make such an operation technically feasible-and economical-are a huge leap from the offshore industry's early days 120 years ago when offshore drilling was done from piers connected to land. The fundamentals haven't changed much, but the technologies tied to the drilling function have improved dramatically over the years.

Drilling fundamentals

Drilling starts with a large diameter hole that is drilled until the formation at the well bottom is near collapse, or the drilling fluid starts leaking out. At that point, a string of pipe, called casing, is lowered into the hole and cement is forced from the bottom, up the annulus between the pipe and the rock.

A smaller drill bit is then used to drill deeper from the bottom of the previous casing. This smaller hole is drilled as far a possible. Then another, still smaller casing, is lowered inside and cemented. This continues until the reservoir is reached. For land and shallow water wells, there are usually four or five casing strings. (See Figure 1).

In deeper water, the number of casing strings may get up to nine or even more. When that happens, some of the casing strings start at the bottom of the last casing rather than from the top of the well. Shorter casing strings are called liners. The last casing string, the production casing, penetrates the reservoir.

The next step is well completion. Part of the well completion operation is to perforate, or blast holes through the production casing. Fluids from the reservoir flow through these holes into the well. The perforation also penetrates out into the reservoir rock, creating cracks that serve as flow paths into the well.

Prior to the perforation operation, a production tree, often called a "Christmas Tree," is clamped to the wellhead at the top of the uppermost casing string. This creates a pressure-sealed path from the top of the well to the production zone at the bottom. The tree is generally rather complex but basically, it is an arrangement of ports and valves providing access into the well.

The last string of pipe, the production tubing is then lowered through the tree all the way to the bottom, creating a flow path for production. It also creates an annulus, between the tubing and the production casing. Production comes up the tubing. Gas and other fluids can be pumped down the annulus for various purposes. Transfer of these fluids from the annulus into the tubing, or out into the reservoir, is controlled by valves that are part of the completion, as described next.

Elements of a downhole completion

The downhole completion generally refers to everything in the hole, below the production tree. The downhole configuration varies widely depending on temperature, pressure, oil viscosity, oil/gas/water ratios, number of production zones, etc. Following are just a few of the elements that might be in a downhole completion.

  • Surface-controlled, subsurface safety valve (SCSSV): Usually a few hundred feet below the mudline, the valve is held open by hydraulic pressure from the surface. Loss of the hydraulic pressure, due to an accident for example, allows a spring to close the valve, shutting in the well completely.
  • Sliding sleeve controls communication between the tubing and the annulus. It may be used to admit gas into the tubing to assist in lifting oil from the hole.
  • Formation isolation valve is used to shut off production from a production zone, for example when that zone begins producing water instead of oil.
  • Downhole gauges are electronic or fiber optic connected sensors that allow monitoring of downhole pressure, temperature, flow rate, etc.
  • Gravel pack is intended to stop the flow of sand coming from the reservoir. Sand in the production stream damages pipe and equipment.
  • Submersible pump is hydraulically or electrically powered, adding pressure when the formation pressure is too low to push product to the surface.

Need for well intervention

Following drilling and completion, a production well comes on stream, that is it starts production. Ideally, production flows, undiminished for years. Of course, that never happens. Instead, production changes. Perhaps the water cut increases and oil production decreases or even stops. Perhaps pressure and flow rates decline faster than expected. Sand content in the flow may increase, eroding piping and pumps. In each case, a diagnosis must be carried out to determine the cause and possible actions that may restore production, or at least maximize it.

Whatever the action, it is called a workover or intervention. The simplest might be pumping acid into the reservoir to dissolve chemicals that are clogging the flow paths in the rock; or pumping high-pressure water to extend the cracks and increase the flow rate. But there is a myriad of other problems that can require intervention—sand, scale or wax collecting in the well; one of the many mechanical valves or gauges failing; tubing corroding and leaking; etc. Some examples are given in Table 1.

Examples of other problems that can require intervention – sand, scale or wax collecting in the well; one of the many mechanical valves or gauges failing; tubing corroding and leaking; etc. Courtesy: EKTInteractive, Inc.

So what is required to perform a well intervention on land or shallow water?

  • Injection: The production tree on a land well sits out in the open at the end of an access road. It usually has a number of hand cranks for valves that shut in production and others for access to the annulus for pumping gas or fluids down the hole. For an injection intervention, a truck hooks up lines to the annulus port on the tree and pumps in the injection fluid. The fluid can be for stimulating the reservoir to increase production. It also might be an inhibitor to reduce corrosion of downhole components or of the tubing. Gas can be injected to mix with and lighten product in the well so it will flow faster.
  • Wireline: The next level of intervention is necessary when the problem is accumulation of sand, scale or wax. In such cases, lowering a tool on the end of a wire or electric line may solve the problem. The tool may be powered, like a mill grinder or pump. This is called a wireline workover and utilizes a lubricator, a long tube through which the wire passes. The lubricator contains the well pressure, collecting the small amount of product that may flow past the wire. Wireline tools can do a range of repairs, including replacing some downhole components. A wireline workover would be categorized as light.
  • Coiled tubing: Well problems often cannot be solved without circulation to remove unwanted fluids or contaminants. A means of quickly creating a flow path to the well bottom is called coiled tubing. Coiled tubing is continuous metal pipe, between 1-1/8 and 1-½ in. in diameter that is wrapped around a large reel. An injector straightens and pushes the small tubing into the well, inside the production tubing. The coiled tubing reel, injector, pump and controls together comprise a coiled tubing unit. They exist in a wide range of sizes and capabilities to meet a range of needs including pumping stimulants, cement, anti-corrosion material, etc.
  • Workover rig: Some problems are beyond the capabilities of wireline and coiled tubing. For example, corroded tubing must sometimes be pulled out of the hole and replaced. In that case, a workover rig, similar to the original drilling rig but smaller, is required. It removes part of the tree and replaces it with a workover BOP. To accomplish this, the well is filled with kill weight fluid, fluid so heavy that it balances the formation pressure at the bottom of the hole preventing product influx. The rig then pulls out the tubing and runs in a new one. Depending on completion design, a workover rig may also be required to replace some of the downhole components. An operation requiring a workover rig would be categorized as heavy.

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