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Optimizing (Enhanced) Oil Recovery – Tune viscoelastic properties of polymers for drilling and recovery

aug 02 2021

The process of oil recovery needs a wide knowledge of engineering and chemistry, it usually follows specific steps. Each reservoir shows its characteristic properties in terms of formation type, porosity, temperature, salinity, and make it challenging to find the right system for the right reservoir. Starting with drilling to completion and finally the different stages of recovery (EOR) use various materials and additives to improve the productivity of the reservoir. Thus, multiple additives must be adapted for each site. Moreover, during exploitation the conditions can change, e.g. porosity or temperature, and reactivity is needed to adjust used materials.

After oil field characterization the Exploitation Steps are: 

  1. Drilling/Drill-in: Drilling the hole rapidly and economically, Reaching reservoir without structure damage, Involves use of drilling fluids, drill-in fluids & additives
  2. Completion: Stabilizing & securing the bore well, requires optimized cement setting protocol to rapidly reach the reservoir
  3. 1st & 2nd stage oil recovery: Use natural energy for recovery (up to 10%) and then maintain pressure with injections (up to 40%) 
  4. 3rd stage oil recovery: solutions are injected to recover up to 60%, multiple additives are used: polymers, surfactants, soaps.. 

Drilling Step

Many of the additives used are gelling polymers that increase viscosity and elasticity, and help to prevent typical issues such as bore well collapse, cut-off sedimentation, loss circulation, etc. One of the key steps start once the oil field is mapped. Drilling is a very complex work and several issues can occur:

> Formation liquid entry - gas and liquids, naturally present in the surrounding rock formation, enter the bore well.
> Bore well collapse
> Drilling bit overheating
> Drilling fluid loss into formation cavities
> Cut-off agglomeration and sedimentation

To avoid above-mentioned issues, drilling fluids are added. They are complex mixtures (aqueous, solvent or emulsions) that help to maintain the pressure and thus prevent the infiltration by formation liquids, as well as bore well collapse. Clay suspensions or other microsuspensions are generally used, as they show a good shear thinning behavior. Drilling fluids need to be fluid during pumping and circulation to fulfil cut-off evacuation, cooling, but as soon as circulation stops, the liquid need to thicken to prevent cut-off agglomeration and sedimentation, but also to maintain pressure to avoid collapse and liquid entry. Rheolaser Master, an optical non-contact measurement, is perfectly adapted to study samples at rest. Additives can tune the viscoelastic properties and parallel measurements allow to optimize concentration and viscoelastic behavior. 

Drill-in is moment when the drilling bit enters the producing reservoir. It is a critical step, as formation damage needs to be avoided. Formation damage is the decrease of the reservoir’s capacity to produce oil. Most often characterized by a reduction of the formation porosity due to cut-off agglomeration inside the pores, clay swelling, precipitation of drilling-fluid salts, …   

Completion

Once the drilling is finished, the bore well needs to be secured to avoid collapse. Amongst others materials (metal, composites,...), cement is used to complete the bore well. The bore well environment is quite complex, and casing materials must be adapted as funciton of temperature, salinity and the completion liquid. Rheolaser Master provide an excellent way to study cement setting under realistic conditions. Moreover, the disposible cells make it easy to study this kind of "dirty" sample, without any further precautions. On the example of different cement-water ratios and the addition of a superplasticizers, the results show, how Rheolaser Master characterizes the cement setting. With increasing amount of superplastisizers, the setting time is delayed and the viscoelastic properties decreases. By variying these parameters, the right setting time and viscoelastic properties can be tuned and improve the completion step. 

 

Recovery

After completion, the well produces oil in the first stage, driven by the natural pressure in the reservoir. This allows to recover up to 10% of the raw oil. In some cases, pumping helps to obtain additional 5%, but it becomes rapidly non-economically. The second stage oil recovery is based on injection of liquids and/or gases to maintain the pressure in the reservoir and recover up to 40% of the raw oil.  

Finally, the third stage oil recovery uses the so-called chemical flooding. This technique of enhanced oil recovery uses different mixtures of alkaline soaps, amphiphilic molecules, to reduce the interfacial tension between water and petrol. This helps to emulsify the petrol, thus, transfer it from the formation rock into the carrier phase. A main issue in chemical flooding are preferential channels through the reservoir, i.e. the flooding liquid finds a way through the reservoir and explore only a small volume of the producing zone. Consequently, the recovery efficiency is low. Using polymers increases the viscosity, even leads to gelation, and stops or slow down the flooding. The solution explores a bigger volume and recovers more oil. The oil recovery is more efficient.

The gelation parameters of the used polymer must be chosen carefully as a function of salinity, temperature, porosity and many more well-related factors. Rheolaser Master offers six measurement positions, temperatures up to 150°C and zero-shear viscosity to study under realistic conditions the gelation of EOR polymers. 

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