The reaction forces in wellhead equipment due to the flow-back of nitrogen foam fracture fluids are analyzed. The forces in the flow-back lines are analyzed for both unchoked and choked lines. The reaction forces are found to be excessive and uncontrollable for unchoked lines with normal field equipment and procedures. The reaction forces in choked flow-back lines are controllable and a safe choke size can be determined for the anticipated wellhead pressure. A new technique for controlling blowdown operations is discussed and preliminary designs for equipment to be used in applying such a technique are given.
Fracturing is a technique for stimulating oil and gas production in formations which have low permeabilities, or in formations where oil and gas permeabilities, or in formations where oil and gas are stored in natural fractures which are not in communication with the wellbore. The use of nitrogen foam as a fracturing fluid offers some very unique advantages over hydraulic fracturing fluids. Nitrogen foam compressed in the wellbore and reservoir becomes an energized fluid. The compressed gas provides energy to remove the fracturing fluid from provides energy to remove the fracturing fluid from the well, also known as the flow-back or blowdown of the well. In such a nitrogen foam fracture operation nitrogen is injected into a well with water, foaming agents, and proppants (proppants are usually a specially screened sand). The mixture is pumped under high pressure (usually 2000 psig to 3000 psig) via specialized surface equipment. Figure 1 shows the typical surface setup for a nitrogen foam fracture operation. Foam fracture operations can have injected nitrogen volumes of 25,000 scf to 300,000 scf. The foam created from the water, foaming agent, and nitrogen carries the proppant down the wellbore and out into the reservoir. As the pressure downhole increases, the fracture pressure of the reservoir will be reached and fractures will begin to form in these reservoir, initiating at the wellbore and propagating outward. As the new fractures are formed, the foam fills them quickly and continues the outward propagation. The proppant is carried with foam into the fractures and proppant is carried with foam into the fractures and will keep the new fractures open after the pressure is released. When the well is blown down, the pressure will be suddenly reduced and rapid expansion pressure will be suddenly reduced and rapid expansion of the gas will force the fracturing fluids from the wellbore. If this is accomplished too quickly, the proppant may also flow back. This is not only proppant may also flow back. This is not only counter productive to the fracture operation, but makes the flow stream abrasive enough to wash out the flow-back system. (The flow-back system is defined as the entire apparatus used in the blowdown operaion; the flow-back line, the wellhead, the valves, the chokes, and the tie-downs for the lines.) For these reasons it is important to control the flow stream during blowdown. After a well has been fractured, it is shut in with the fracturing fluid still in the wellbore. This allows the fractures to 'heal' around the proppant so that the proppant doesn't flow back out proppant so that the proppant doesn't flow back out of the fractures when the pressure is released. During the shut in period some of the fracture fluid, and, therefore, the pressure, will bleed off slightly. The shut-in period is usually from 1 to 6 hours. Prior to placing the well back on production, the Prior to placing the well back on production, the fracturing fluids must be cleaned from the wellbore. This is the blowdown or flow-back operation. During the shut in period, the flow-back lines will be connected to the wellhead. These lines are generally line pipe or tubing 100 feet to 300 feet long. The flow-back system will allow fluids to flow from the wellhead to a waste or burn pit. After the shut in period, the wellhead valves are opened and the energized fracture fluid and some proppant are allowed to flow to the pit. As pressure is reduced, the nitrogen expands providing energy to assist lifting the fluid to the surface. At this point, this energized fluid possesses a large amount of energy which must be controlled throughout the flow-back system to prevent breakage of lines and accidents.