Injection of heat-depleted brines into clastic sedimentary reservoirs combining alternating clay, sand and sandstone sequences has long been reported as a sensitive matter among petroleum and geothermal operators.

The operator of a geothermal plant has to pay special attention to the reinjection of the cooled-down thermal water into the respective porous and permeable layer, as economic operation is guaranteed only when the thermal water can be reinjected over longer periods of time at little expenditure of energy, i.e., no blockings occur in the injection horizon.  

Originally, reinjection serves for disposal of waste water, recently however, it has become an important mean for reservoir management - injection of cooled-down geothermal water back into the producing formation is a procedure that maintains reservoir pressure and increases energy extraction efficiency.

At the same time, safe reinjection of low-enthalpy geothermal fluid into sandstone has not been successful for a long time, and has set a very serious professional problem impeding the utilization of geothermal energy.

First tests for reinjection into sandstone began in the early 1980s in Germany and France, hitting against serious difficulties. Reinjection will namely lead to severe well and formation damage when dealing with fine grained, matrix type, more or less consolidated clastic deposits including sand, sandstone and clayey interbedded sequences as encountered in detrital continental, and fluvio-deltaic sedimentary environments which take an important share of the European geothermal resource.

As a matter of fact fast plugging kinetics have been experienced in several injection wells drilled in the early 1980’s in Paris basin triassic sandstone which caused these objectives to be abandoned. In those cases well impairment was attributed to odd completions, particle invasion and gas entrapment among others. At the same time, the utilization of reinjection is required for the sustainable, environmentally friendly exploitation of geothermal energy in the South Great Plain.⁷ 

The main difficulty of reinjection into sandstone is the poor permeability. Well and formation impairment caused by water injection is a consequence of one or several of the damaging factors listed below:

1. chemical incompatibility between injected and formation (native) fluids;

2. microbiological effects;

3. water sensitivity of sandstones;

4. suspended solids (fines, corrosion products, scale);

5. fines migration within the injected formation;

6. trapped gases;

7. air contamination;

8. non compatible chemical additives and inhibitors;

9. thermodynamic changes induced by the injection process;

10.  injection flow rates;

inadequate well completion.

Investigations into clastic sediments show that the potential injectivity index (i.e., the thermal water injection flow rate which is possible at a certain pressure) can be concluded from the results of the geo-scientific investigations and the production tests.For the selection and assessment of such a site, geological and geo-hydrologic parameters of the underground as well as the adequate dimensioning of the drilling installation, of the surface loop and the economic profitability of the entire plant have to be properly considered.  

As stressed by various authors, the precise mechanisms, which make injectivity indexes drop, are not entirely understood. As a result, carefully designed and operated field tests and laboratory experiments on formation cores are required to secure reliable water injection programmes.  

With both international and domestic examples proving that the injection of thermal waters into porous reservoirs is technically manageable, it is a fact that neither a practical know how, nor a professionally grounded and formalized policy is available at the moment, with the use of which the success of projects applying reinjection into sandstone can be guaranteed.