The well completion is the lowermost portion of the well, comprised of tubulars and downhole equipment, that enables the safe and effective production from an oil or gas well. The objectives of the completion are to:
- connect the reservoir to the production tubing;
- provide a conduit for well stimulation fluids;
- protect the well from sand/fines production;
- isolate productive zones from non-producing zones;
- isolate oil producing zones from high GOR (gas-oil ratio) and high watercut zones in oil reservoirs;
- isolate gas producing zones from high GLR (gas-liquid ratio) zone in gas reservoirs;
- provide a means to measure changes in reservoir conditions (pressure, saturation, rate) by well tests, cased-hole logs, and production logs.
Before we can discuss well completions, we will need to discuss sources of production problems. The problems that we will further discuss in this lesson are production of sand and production of unwanted fluids (gas and water from oil reservoirs and water from gas reservoirs).
We have already discussed that in reservoirs made of unconsolidated reservoir rock or in reservoirs with fine rock materials, there is a possibility that the rock material and debris may be dragged along with the produced fluids and enter the well. As I mentioned earlier, this this is referred to as Fines Movement or Fines Migration. Once inside the well, if the velocity of the fluids going up the well is less than the settling velocity of the sand, then Sand Fill will occur in the well. Sand fill is the settling of sand and debris originating from the reservoir which entered the well and settled on the bottom of the well. This has the potential to cover some or all of the perforations in a perforated well. This is illustrated in Figure 7.04.
In this figure, sand fill has covered approximately half of the perforations and is severely restricting flow from the reservoir. The ineffective perforations and the restricted flow shown in Figure 7.04 result in well damage and is quantified as a Skin Factor in our Inflow Performance calculations.
If the velocities of the fluids are greater than the settling velocity of the sand, then produced sand will continue upward and cause erosion of the tubing and possible damage to any down-hole equipment (down-hole pumps, pressure gauges, etc.). The detection and avoidance of sand production is referred to as Sand Control.
Another potential problem in hydrocarbon production is the production of unwanted reservoirs fluids. As petroleum engineers, we are attempting maximize the recovery and profit of a well for our employers and key stake holders. To achieve this, we are interested in producing the most valuable resource from the well. In the case of oil reservoirs, this means the production of oil. While natural gas is also produced from oil reservoirs and has a sales value, crude oil has historically commanded a higher commodities price than natural gas. Consequently, production engineers working on oil reservoirs will focus their attention on oil production and, in fact, in many cases attempt to shut off gas production.
As we have seen in earlier lessons, free gas in the reservoir (either liberated solution gas or initial free gas) expands and displaces oil to the production wells. This expanding gas is the cause of two of the oil drive mechanisms: solution gas drive (liberated solution gas) and gas cap drive (initial free gas). From the reservoir side, the production of the free gas results in removing a source of reservoir energy available to oil production.
From the tubing side, the production of free gas can have different impacts based on the production rate of the free gas, At low rates, the addition of gas will lighten the well column and aid oil production. On the other hand, excessive gas rates may result in higher frictional losses and, in turn, result in a high back-pressure across the perforations. From our discussions on inflow performance and Darcy’s Law, we saw that the lower that we can keep the flowing well pressure, the greater the drawdown will be and the higher the oil production rate will be. It is the role of the production engineer to assess the impact of gas production (both from the reservoir side and the tubing side) to determine if/when gas shut-off is required.
For both oil reservoirs and gas reservoirs, the production of water is always avoided if economically warranted. Since water is a heaver fluid than either gas or oil, water production results in a heavier well column and a larger back-pressure across the perforations. In addition, depending on the produced water chemistry, it may need to be treated to remove any heavy metals and disposed of. Water treatment and disposal require additional costs which reduce the economics of the well.
The detection, avoidance, treatment, and disposal of water is referred to as Water Control. The best method of water control is to shut off water in the reservoir. There are three common causes of production of unwanted fluid production. These are:
- lateral fluid movement from a gas cap or aquifer,
- lateral fluid movement from gas or water injection, and
- gas and water production from coning.
These mechanisms are shown in Figure 7.05 in the context of water production. In Figure 7.05 (A), water is being pulled laterally from the aquifer by the production from the well. The movement of water from the aquifer to the well is, in large part, governed by the permeability of the different reservoir layers with the water moving faster in the reservoir units with the highest permeability. In Figure 7.05 (B), water is being injected into the reservoir. The objective of this secondary waterflood is to displace oil to the production well with the injected water. At some point, the injected water Breaks Through to the production well resulting in increased producing watercuts. Finally, in Figure 7.05 (C), water is being pulled upward from a underlying or Bottom Water Aquifer. The production of water in this manner is referred to as water coning.
Each of these water producing mechanisms has a direct analog with a gas production mechanism. These gas production mechanisms are shown in Figure 7.06. The avoidance of production of sand and unwanted fluids are two major design criteria for well completions.
There are many types of completions, however, we will focus on the following:
- an Open-Hole or Bare Foot completion
- a Screen or Liner completion
- Slotted Liner completion
- Screen completion
- Gravel pack completion
- Cased and Perforated completion