Different companies have different policies. In some exceptionally sensitive areas or situations, there could be restrictions, but wells are commonly flowed and flared during the night. Many companies have the restriction to allow first hydrocarbon to surface only during the day light hours. Some companies further refine it by type of well like wildcat, exploratory, development, onshore, offshore etc. The basic premise behind this policy is the level of associated risk.

Since the reservoir fluid properties are conclusively ascertained after collecting sample at surface, there could be higher level of risk in terms of ingredient of reservoir fluid that makes it to the surface and readiness of the crew to handle unexpected conditions like high H2S percentage etc. Another consideration is the possibility of spill causing environmental risk. Although all the surface equipment and flow lines are tested before flowing the well, there could still be surprise leaks.

The risk should be assessed on case-by-case basis and most effective, efficient and ALARP risk-based option should be selected. After evaluating risk scenarios and mitigation measures, residual risk level should be assessed. If the team feels that considering the knowledge of the field & reservoir, lighting at rig site and readiness of crew, any such surprises can be handled safely, then the possibility of requesting a waiver should be evaluated.

Stimulation and Fracturing both aim to enhance well productivity, but they vary in application. Stimulation, sometimes also called acidization, commonly refers to acid injection in the well to either reduce the effect of formation damage or create wormholes in the near wellbore area to increase productivity. Fracturing, in broader terms, is the method of stimulating a well through mechanical means to produce more. The selection of an appropriate approach depends on many factors, such as reservoir characteristics, depth, reservoir potential, environmental considerations, cost, etc.

Matrix Stimulation or acidization is a process of pumping a designed acid treatment train into the well to reduce wellbore skin. Depending on the formation and the objectives, a combination of acid or solvent fluid is pumped into the wellbore in a predefined sequence and volume, keeping the pressures below the fracture pressure. The matrix stimulation is used in carbonates as well as in sandstone with different objectives. In sandstone reservoirs, the treatment aims at improving productivity by cleaning and enlarging the near-wellbore channels and pore spaces. A combination of preflush, main treatment, and overflush improves formation permeability in the near-wellbore area by removing the formation damage and dissolving the material plugging the pore spaces. In carbonates, the process aims at creating new channels, also called wormholes, that extend beyond the damaged area. Hydrochloric acid (HCl) is commonly used as a treatment fluid for carbonates.

The effectiveness of acid injection depends on acidizing parameters such as injection rate and volume. If the rate is low, it results in surface dissolution in the near wellbore area. A high-rate injection is required to form wormholes. Simulation of the optimum injection rate predicts the wellbore-pressure profile and wormhole distribution by tracking the movement of the acid in the wellbore. The simulation takes into consideration several factors like the well profile, depth, lateral length, formation type & properties, etc. Long horizontal sections in carbonate reservoirs require effective acid distribution along the entire reservoir length. They may need large volumes of acid injection at high rates to form effective wormholes deep into the reservoir.

Fracturing is the process of fracturing the rock bed with pressure. It is primarily used in low-permeability reservoir rocks. Due to low permeability, the fluid-containing pores are not able to connect to the wellbore. The process of Fracturing creates new cracks in the rock, which act as conduits for the flow of reservoir fluid into the well. Fracturing could either be 'Explosive Fracturing' or 'Hydraulic Fracturing.'

The process of 'Explosive Fracturing' utilizes explosives to fracture the formation. The high-pressure gas generated through explosion exceeds the fracture pressure of the rock and creates new fractures. The job is designed based on simulations to evaluate the effect of different explosive quantities on crack fractal dimensions under varying values of confining pressures. In most cases, under the same confining pressure, cracks increase with the increase of explosive quantity. Although this relationship could be approximately linear under relatively low confining pressure, however exhibits nonlinear behavior when the confining pressure is relatively high. Hence it is important to carry out a proper simulation before the job to enhance the effectiveness of the fracturing process.

Hydraulic Fracturing, also known as 'Fracking,' is the process of injecting 'frack fluid' into the formation at high pressure. The fracking fluid is a mixture of water and sand or other proppants. Proppants are kept in suspension in the fluid by adding a thickening agent. As the fluid is pumped into the well, it increases pressure due to resistance from the formation. Once the pressure exceeds the breakdown pressure, which is the sum of in-situ stress and formation strength, fractures are created in the formation. The natural tendency of the fractures is to close under the overburden pressure as soon as the injection pressure is removed. However, the proppants in the frack fluid are lodged inside newly created fractures and do not allow them to close. These fractures connect the pore spaces and natural fractures in the reservoir to the wellbore and allow the formation of fluid to flow into the well for production.

PerfFRAC is a combined perforation and fracturing technique used by Schlumberger. PerfFRAC is an improvement on a field-proven technique originally developed by ExxonMobil URC. Depending on the reservoir conditions, the PerfFRAC technique provides a cost-effective and efficient solution for perforating, fracturing, and comingle flow of multiple zones in a single run.

The conventional approach for completing multiple zones involves the following steps:

• Perforate the lower zone.

• Pump fracking treatment for the lower zone.

• Flow back the well overnight for clean-up.

• Isolate with a composite bridge plug to protect the zone.

• Perforate the shallower zone.

• Pump fracking treatment for the shallower zone.

• Flow back the well overnight to clean up.

• Isolate the second zone with a composite bridge plug to protect the zone.

• The process continues until the uppermost zone has flowed back.

Once perforation, fracturing, flow back, and proper isolation of all zones are accomplished, coil tubing is run to drill out all the composite bridge plugs set to isolate different zones.

• The well is then flowed back to clean up for completion.

This approach does ensure proper fracturing and isolation of every zone but needs multiple wireline and coil tubing trips requiring multiple days to finish the process for all the zones.

The PerfFRAC technique achieves all these objectives in a single run through ‘Select Firing’ guns and ‘Ball Sealers.’ The ‘Select Firing’ technology allows running multiple perforating guns together in one run and activating them one at a time. The process is carried out in the following broad steps:

• Run multiple select firing guns on the wireline to perforate all zones.

• Position and fire the perforating gun across the lowermost zone.

• Bullhead fracking treatment for the lowermost zone.

• While pumping the treatment, pull the wireline to position the perforation guns across the next zone.

• Pump displacement fluid (flush) to tail the fracking treatment.

• Add ball sealers with the displacement fluid.

• Observe an increase in pressure, which indicates the sealing of the open perforations with the balls.

• Perforate the second zone.

• Bullhead fracking treatment to the second zone. The ball sealers isolate the lower perforations, ensuring that the treatment targets the newly perforated zone.

• Pull the wireline to position guns across the next zone.

• Add ball sealers with the displacement fluid for the second zone.

• Observe an increase in pressure, indicating the sealing of the open perforations with the balls.

• Perforate the next zone.

• The process continues until all zones are perforated and treated.

• Flow back the well to clean up for completion.

Ball sealers dislodge from the perorations due to differential pressure during flow back and are captured on the surface in the ball catcher. Some ball sealers, which do not surface, safely fall into the rat hole with no adverse impact.

Perforating & fracturing in one run using this technique provides significant time reduction and cost savings.