\ \ \ ~ \ \ \ \ \ \ \\

B

BUBBLES

Cement

Figure 41.-A "kick" is a gas or liquid influx that reduces the hydrostatic head in the annulus. Here, the kick is a gas bubble (A). As it rises (B and C), it expands--causing a sudden increase in the upflow of the mud. When the bubble reaches the top, if it has not been allowed to expand, the bottom-hole pressure reaches a maximum -- the sum of mud pressure and gas pressure. This pressure maximum, if excessive, can exceed the formation fracture pressure, and lead to a loss of drilling mud to the formation, thus further decreasing the hydrostatic pressure. This could cause an influx of formation fluids into other formations, or the fractured formation taking fluid from another formation, commonly referred to as an underground blow out.

(Adopted from Panel on Operational Safety in Offshore Resource Development, "Outer Continental Shelf Resource Development Safety", Marine Board of National Academy of Engineering, December, 1972).

preventer system would inlcude the following:

(1) one

bag-type; (2) one with blind-rams; and (3) one or two with pipe rams, depending on how many strings of casing have been set in the bore hole. Blowout preventers are essentially large valves that can close around the drill string or across an open hole and seal off the well at the surface. Blowouts can occur downhole when a low-pressure formation fractures, and fluids from a higher-pressure zone flow into the fractured formation. Such underground blowouts, like surface blowouts, require the careful use of preplanned emergency techniques to regain control. Blowout preventers and other well-control equipment must meet the requirements of OCS Order No. 2. This equipment is tested on a schedule set by prudent practice, but not less often than regulations specify.

To ensure that adequate provisions have been made for safety and well control, the casing program and drilling mud program must be approved by the Geological Survey before a drilling permit is issued. Along with adequate casing, it is important that enough cement be spotted between the casing and the wall of the hole to seal off and isolate all sensitive geological formations such as hydrocarbon zones and fresh water sands, and to separate abnormally pressured zones from those with normal pressures. A prime function of the drilling mud is to maintain hydrostatic pressure control in the well, and mud is tested frequently during drilling operations to ensure that it has sufficient density and meets other physical and chemical specifications.

b. Well Completion Phase

Should the initial test well be dry, it is plugged with

cement. One objective is to prevent the escape of formation fluids from their original reservoirs either into other formations or to the surface. When a well is abandoned, the casing is cut-off at least 15 feet below the mud line, all obstructions are removed, and the bottom is dragged to be sure that no obstructions were overlooked. plugging operations, well-control equipment remains in use. In some cases, it may be necessary to drill several exploratory wells on the 5,760 acre block before the lease is finally condemned.

During

Fluids from formations penetrated by wells are often brought to the surface in drill-stem tests to evaluate the possibility of oil and natural gas production. These fluids are collected in tanks at the surface; drilling mud is separated from the produced fluid, and if the formation fluid is oil it is either stored for later disposition, or the oil and natural gas are flared in specialized, high volume burners.

If well tests show that commercial quantities of natural gas or oil have been found, it may be necessary to drill and evaluate several additional confirmation wells before the company is satisfied that the reserves will support installation of a drilling-production

platform.

It is also important to delineate precisely the extent of the petroleum reservoir because of the great expense of deeper water platforms and

the economic necessity of drilling as many production wells as possible

(sometimes over 30) from a single platform.

Platform location in relation

to hydrocarbon deposits must be extremely accurate to minimize the number

of platforms installed.

If petroleum deposits prove to be of commercial size, one of two courses

of action may be followed:

(i) The exploratory well may be deemed expendable and Procedures followed would be the same as in

be permanently abandoned.

dry hole abandonment.

(ii) The well may be deemed satisfactory for future use In this case, a

as a production well and temporarily abandoned.

mechanical bridge plug is emplaced in the smallest string of casing and

the well head capped and left for future entrance when production activity commenced.

This results in the temporary existence of an

underwater "stub". The Coast Guard District Commander requires that

such stubs be marked by a buoy at the surface if located in 200 feet of water or less, and that the buoy be lighted if located in 85 feet of water or less.