Underbalanced drilling becomes very famous in several places of the world because there are some advantages over normal drilling operations. Today we would like to share the basic underbalanced drilling 101 for everybody who is interested in this topic. It is presented by Shell and we also add full VDO transcript for anybody who cannot catch the information from this presentation.
Underbalanced Drilling 101 Full VDO Transcript
This presentation introduces the concept and equipment used in underbalanced drilling operations. The key differences between underbalanced drilling and conventional overbalanced drilling are at both the conceptual and the technical level. At the conceptual level the subsurface drilling conditions needs to be investigated further, the rotating drill bit cuts away rock to deepen the well. The drill cuttings are lifted to the surface by the circulating drilling fluid.
In conventional overbalanced drilling operations, the hydrostatic pressure exerted by the drilling fluid in the well is designed to exceed the pressure of the hydrocarbon fluids in the reservoir. Since the pressure in the hole is higher than the pressure in the rock fluid therefore drilling fluid can lose into the formation. These losses cause damage to the near well-wall area resulting in reduced production, experience show that even a short exposure to overbalanced conditions can severely impair the productivity of the well.
In underbalanced drilling operations, the hydrostatic head of the drilling fluid is designed to be less than the reservoir pressure. This means that there is a continuous flow of hydrocarbons into the well during the drilling process. Under this condition, no near well-wall damage occurs and the wells ultimate production is not impaired. The privation of well-wall damage has several advantages; near well-wall damage prevention is just one of the benefits of underbalanced drilling. Another benefit of underbalanced drilling is the reduction of drilling problems such as differential sticking and drilling fluid losses. Producing the well while drilling increases reservoir knowledge, with this knowledge wells can be steered into more productive zones.
Reservoir pressure and depth determine the required density of the drilling fluid system, unlike conventional drilling which is limited to simple fluid systems, underbalanced drilling uses a variety of fluids to control bottom hole pressure at the high end of the density scale light fluids such as water syntactic based oils, natural crude oil or diesel form the drilling fluid.
To achieve lower bottom hole pressures a two phase flow system can be employed. This system consists of a light fluid aerated with a gas such as nitrogen or natural gas. First systems generally consist of water and surfactant. At the low end of the scale, mist or air based systems can be employed. Each of these fluid systems requires a different surface setup in what follows the commonly used two phase flow system is shown as an example to illustrate the technical differences between underbalanced and conventional overbalanced drilling.
A two phase flow system setup consists of the following elements, the light drilling fluid is stored in the rigs mud tank system and is pumped to the well by the rig fluid pumps lifting gas, in this example nitrogen, is injected into the fluid stream. Depending on the quantity of nitrogen needed. The source of the gas can be cryogenic tanks or a membrane unit that separates nitrogen from the air. The two phase fluid stream is injected into the drill stream using the conventional rig systems. The two phase fluid exit the base of the bottom of the hole carrying drill cuttings up the annulus. Since the hole is drill underbalanced reservoir fluid such as oil or gas are produced from the reservoir and mingled with the two phase fluid moving up the annulus, this means that the well is producing while being drilled. The returned fluid from the well is diverted by a rotating control head to the surface separation equipment.
The control head seals around the drill pipe while allowing the pipe to move in and out of the well and rotate. In some cases a snubbing unit needs to be installed to allow movement of the drill pipe in and out of the well under pressure. Moving slips on the snubbing unit grip the pipe and push or pull it against the upward force in a controlled manner. Flow from the well is controlled by a choke manifold and the processed in a full phased separation system, this system separates the returns into its constituency, light fluid, crude oil, nitrogen and natural gas and rock cuttings. The separated cuttings are collected for the disposal. The separators hydrocarbons are sent on the production facility flare. The light fluid is pumped back to the rigs mud system for treatment and reuse. This completes the tour of the main surface systems that are used in two phased underbalanced drilling operations.
In summary underbalanced drilling allows the well to product during the drilling phase. Underbalanced drilling operations require additional surface equipment and a careful upfront design. When applied in appropriate wells underbalanced drilling prevents formation damage reduces drilling problem and increases reservoir knowledge.