3.0WELL KILLING AND ACTIVATION PROCEDURES:

3.1Killing of wellAll the exploratory, development and work over wells need to be subdued/ killed before taking up any activity in the uncontrolled condition/status of well. Once the well is completed and ready for production, the well needs to be activated for resuming production/ injection. As a standard practice, the well are killed by the following procedure(s)

1. Higher gravity killing fluid (Mud, Brine etc.) to counter balance the reservoir pressure.

2. When the killing fluid flows back into the reservoir due to high permeability of zone or pressure imbalance between reservoir pressure and killing fluid, knocking out of killing fluid from the well for activation needs more time and efforts. Sometimes, the well does not come backs to its original potential. In such wells, carbonate pills are mixed with the killing fluid and placed opposite to the perforations so as to stop this phenomenon. In case the return permeability/ production rate is not achieved during activation, the pills are dissolved by treating zone with light HCL acid and activation process is affected.

3.2Well Activation:All Exploratory, Development and Work Over wells are put for activation to acquire productivity data to prove the presence and potential of hydrocarbons in the reservoir & produce them optimally for maximum recovery from the well. Lot of time and money has already been spent till release of location, drilling and conventional completion of well. Appropriate completion design keeping logical speculation for forthcoming stimulation requirement in near future, shall result in reduced activation/ testing time in addition to correct evaluation of formation for optimum/sustained production from the field. Unsystematic approach towards completion design for initial testing/ stimulation requirements and acquisition of intermediate and final reservoir data, may be disastrous as a field may be lost or a sick self producer is created or an inefficient artificial lift may be installed. Similarly, inappropriate well killing/ activation procedures may lead to similar fate of the well.

Most of the wells in matured fields are sub hydrostatic and candidate for installation of Artificial Lift even at the initial stage of production.

For activation of wells, following methods are being adopted worldwide:

1. Application of high pressure air compressor:

2. Application of gasified liquid nitrogen gas

3. Temporary gas lift installn with down hole packer and application of air compressor/ nitrogen gas.

4. Application of CTU

5. Application of surge tool

3.2.1 & 3.2.2Nitrogen gas or high-pressure air (by air Compressor) is applied in the casing. Liquids are U-tubed and brought out through tubing on the surface. In this process, if the pressure (head) in casing {Liquid + Air/N2} against the perforation (well bore) is higher than the reservoir pressure, a situation of reverse flow to reservoir or No- Flow of liquid to surface may result. This is generally encountered in high permeable reservoir. This practice is not good for health of reservoir.

In case of low permeable reservoir, the good quantity of liquid is seen on application of Air compressure/N2 at first instance and subsequently when the gas cut/ air cut is affected, the whole annulus and tubing volumes are discharged on the surface. The Gas/ air injection is stopped; annulus gas is flown back and discharged to the atmosphere. This process creates high draw down at the well bore. This situation may result in sand incursion or blockade by fine sand near the well bore. This results in reduced productivity. Application of Nitrogen/ Air without down hole packer, the formation encounters inflow of drilling/ killing fluid from annulus towards the formation. In case the fluid inside the annulus/ casing is not clean and having suspended particles, the particle with flow will enter the pores and get deposited under air/ gas pressure. This will be damaging/ reducing the flow path of the actual reservoir fluid as the deposition of unwanted materials present in the killing fluids, may take place on rock/sand.

Poor producers take long time to load the well in tubing and casing with formation liquids. This phenomenon results in wastage of costly rig time and uncertainty of time required for second / subsequent applications of Nitrogen/ High pressure air.

3.2.3 Therefore, in order to avoid such losses i.e. loss of permeability/ production due to entry of suspended material in formation, installation of Packer with gas lift system on tubing or retrievable GL mandrill will reduce the time for rise of well liquid in the tubing and hence reduce the time interval for second/ subsequent Nitrogen application for activation of well. In cases where PI is good, the Air/N2 application may be in continuous mode. This practice shall Tremendously reduce the rig time for activation and save on repeated deployment and untimely application of costly inputs like high-pressure Air/N2.

Figure 3.2.3

Hydro-Frac Wells not completed with hydraulic packers & GL System: During Frac operation, substantial quantity of liquid (about 120 Cubic Meters with Gel) is injected in the zone of interest. It takes very long time and number of nitrogen/ air compressor applications to flow back all the injected frac fluid. Moreover, high draw down is not at all recommended in frac well. When Frac Wells are completed with down-hole packers but without Gas Lift Valve on the tubing string, the activation process shall require Nitrogen gas application through Coiled Tubing and in this process high/ uncontrolled Draw downs are faced by the reservoir. This phenomenon, in some cases, may result in inflow of frac sand. This phenomenon damages the permeability created by frac operation and also partial or complete blockage of the perforation intervals.

Installation of down hole packer and Gas lift valves with reverse flow check valve on the tubing is the solution to reduce the activation time, assessment of initial productivity of well and repeated deployment of Coil Tubing Unit and Nitrogen Pumper for activation of wells.

In order to combat with the above problems encountered during activation, the following actions are advised to complete the well for achieving prolonged and sustained production from the low producers as well sand prone wells:

The tubing shoe should be above the top of and close to perforation.

1. Hydraulic packer must be installed one tubing above the tubing shoe.

2. Gas lift valves and mandrels must be installed considering Air compressors maximum pressure generating capability. Last- bottom most mandrel must be installed with reverse flow check valve (without gas lift valve) to act as circulation valve. First/ top valve, known as unloading valve to be as per available maximum pressure generating capacity of Air compressor and killing liquid and Air specific gravities. The gas lift design must be of Continuous Lift Mode. However, while unloading the well, a correct time interval may be evaluated for second application if the flow of liquid from the well stops and only gas/ air outflow is seen. This implies that the continuous mode gas lift design may be converted in to intermittent flow pattern even without pulling out/ running in of completion string with packer and gas lift valves etc.

3. In cases when productivity of the wells/ reservoirs is low, the inflow of liquid will be less than the liquid requirement/ lifted by continuous gas lift design. The situation shall come when only air will be coming out of the tubing and no liquid is seen. At this stage, the air injection must be stopped and suitable tools must be lowered in the well to acquire data like SBHP, Build up curve etc so that PI values are evaluated to decide mode of Artificial Lift (SRP or Gas Lift) and its size (design parameters) to produce the well optimally for longer period and improved recovery factor.

4. In case there are indications of presence of potential hydrocarbons during the process of Air application during activation of the well, the air injection must be stopped and the air bled off. Nitrogen must be applied in place of Air for safety reasons.

5. Once the parameters are evaluated, the desired mode of lift may be designed and installed on the well either now when rig is at location or subsequently when other infra structure requirements for putting the well on artificial lift i.e. electrical supply line or gas injection line as the case may be, are created/ completed to save on costly rig time and also the well could be put on production immediately after installation of Artificial Lift with optimum production rates.

The suggested completion shall avoid back flow of killing fluid in the formation safeguard the reservoir against stresses arising due to compression and sudden evacuation during the process of compressor application and unloading/ Activation of the well. There will be a uniform draw down and some head against the well bore which will avoid the vacuum, chances of sand incursion and control the liberation of gas from oil inside the reservoir itself. The correct PI values shall help in deciding and designing of correct mode and size of artificial lift. This will give trouble free, prolonged and sustained production.

3.2.4In case, the well completion is equipped with down-hole hydraulic packer but without gas lift valves, the activation is done with the application of Coiled Tubing and Nitrogen application thru CTU. The unloaded quantity is only the tubing volume in one application. Though the rise in level in the tubing will be faster as compared to when well is completed without Packer, these units (CTU and N2 Pumper) have to wait at site till the well is ready for second application as the Rate of liquid inflow is not certain and these units are to attend other rigs/locations also.

Fig 3.2.4

3.2.5 WELL ACTIVATION USING BACK SURGE TOOL

Various methods of well activation have been used in development wells with mixed results. Activation of highly deviated wells such as ERD wells or horizontal wells poses challenges that require special tools or innovative ideas to successfully activate or stimulate these wells.

Some of the methods of activation in these types of wells are:

1Matrix acidization job

2Nitrified MAJ

3Acid spotting with CTU

4Acidization through CTU

However, the results from the application of these techniques were not very encouraging.

Recently, many such wells have been activated through use of Back surge tool (Figure 2.26) with excellent results and most of wells so activated have not required stimulation jobs.

The formation surge tool provides an instantaneous draw down across the formation that assists in the removal of debris from the area around the perforated tunnel.

The tool is run on a lock mandrel to an appropriate landing nipple profile. Precisely rated shear screws are used to set the tool to activate at a predetermined draw down pressure starting from 500 psi to 4000 psi.

Activation of the tool is achieved in a number of ways:

Bleeding off above the tool

Circulating out well contents to nitrogen above the tool

Pre-pressurizing the formation and bleeding off above the tool.

Figure 3.2.5 Back pressure surge tool