There are many advantages associated with open hole completion designs. Asset teams may save on time and costs as there is no need to case the entirety of the borehole. They may also benefit from having to use fewer materials, equipment, and operations. With open hole completion designs, inflow is also less impeded so more hydrocarbons can be recovered, faster.
While advantageous, the viability of open hole completion designs largely depends on the geology of the oil field. The pressures exerted by the weight of the earth closes wells bored into softer formations. The flow of water or brines also displaces fluid from the wellbore, or introduces impediments like mud or cuttings that block the flow of fluid. Efficient and accurate visualization of the geology in oilfields and individual pay zones is critical in the success of open hole well completion designs.
The Advantages and Disadvantages of Open Hole Completion Designs
The chief advantage of open hole completion designs is fewer operations need to be performed before a well begins producing. The time and cost savings can be considerable. There are also some additional advantages to this sort of well design when it comes to recovery operations.
- Since there are fewer operations being performed to prepare the bore, the potential for damaging geological formations in ways that impede production is reduced.
- The interior diameter of the bore is larger which results in more consistent production pressures.
- Since the reservoir zone is left fully exposed allowing hydrocarbons to flow unimpeded the volumes of products that are recovered, and the speed at which they are recovered may be greatly improved.
- Open hole completions may be preferred in horizontal wells where casing the entire length of the horizontal section may be technically infeasible or too expensive.
The reduced wellbore coverage that lends open hole completion designs these advantages is a double-edged sword. The reduced casing on the wellbore lessens the predictability of production, and the control the operator has over it. Open hole completion designs have the following disadvantages:
- The inability to isolate hydrocarbons, or manage production for different zones in a reservoir
- The inability to control excessive gas or water production
- The larger diameter allows more consistent pressures, but the velocity of product is lower
- The potential to produce sand is high with open hole completion designs
- Open hole completions in vertical wells are usually not feasible when there is low formation integrity.
The disadvantages stem from the same source as an open hole completion design’s advantages. Leaving the borehole unclad allows for improved flow of hydrocarbons, but can also allow the flow of undesired fluids or materials to wash into the borehole or the reservoir. The nature of an open hole completion design is limited by geology. The design is only viable in reservoirs where the surrounding geology has the strength to keep the borehole intact, doesn’t allow for fluid flow to move significant amounts of soil into the well, and where the reservoir allows predictable recovery of hydrocarbons by volume and type.
The Importance of Data in Determining Geology
Open hole well completion designs are often limited to limestone reservoirs and “dry” shale reservoirs. Overall these formations are most likely to meet the requirements for successful open hole completion designs. These are hard, well-consolidated reservoir rocks with largely predictable resistivity. However, the presence of limestone or shale formations on their own is not sufficient to determine the viability of an open hole completion design.
Geological strata are layered, as are the hydrocarbons contained in geologic reservoirs. Otherwise, suitable reservoir formations may be near gas or water reserves that could interfere with production through a variety of different mechanisms. Detailed geologic models are of importance in identifying suitable formations for open hole completion designs, and also identifying fluids and looser unconsolidated strata they may intervene in the drilling zone and render the site unsuitable for this type of well completion. Visualizing this data is key in developing well completion designs of any type, but due to open hole completion designs’ reliance on existing geology, it is especially important in drilling these wells initially and in ongoing well and reservoir management as they operate.
The Benefits of Detailed Visualization of Geologic Models
Initial surveys generate seismic data that can be used to determine the rough extent and depth of geologic formations. These can be further refined by boring exploratory wells, using seismic sensors to create vertical profiles, and even measuring gamma rays to more accurately determine underlying strata consistency. This data can be further refined during oil recovery, and changes occurring across time can grant further insights into the exact layout of the geology in an oil field. Logs of the production pressures and volumes of the wells can reveal changes in the well, or in the resistivity of the surrounding geology that could indicate that sands or fluids are intruding into the borehole.
Aggregating all available data into a single visual model is the most reliable way to develop efficient and accurate open hole completion designs.
Aggregating all available data into a single visual model is the most reliable way to develop efficient and accurate open hole completion designs. They are the most effective way to detect fluid or non-consolidated strata in an otherwise suitable reservoir formation. Updating these models over time with further refinements from the oil and gas data generated during production can reveal the suitability of geology for open hole completion designs in a petroleum field.