Stimulation of petroleum producing formations has accounted for a large percentage of the current producible petroleum reserves in many countries. This technique can assist in obtaining more producible reserves.

Improvement of productivity and injectivity of wells started with shooting wells with nitroglycerin; later acidizing of calcareous formation was introduced and hydraulic fracturing for stimulation became commercial in the late forties. Extremely low permeability formatiorts are now producing economical quantities of hydrocarbon as a result of massive hydraulic fracturing treatments.

Fluid, proppant, equipment and technology have been developed to treat not only shallow wells but also deep, high pressure, high temperature wells. Many papers and talks presented at this the 10th World Petroleum Congress have addressed the need for more petroleum.

Worldwide there are reservoirs containing large volumes of hydrocarbon in place, but they cannot be produced at economical rates. Methods of increasing the productivity of these reservoirs possibly can make them economical. Papers presented touched on several facets of increasing well productivity.

The first paper was 'Stimulation of Asphaltic Deep Wells and Shallow Wells in Lake Maracaibo, Venezuela', by C. Von Albrecht and B. Diaz, Venezuela; and W. M.

Salathiel and D. E. Nierode, USA.

Von ALBRECHT showed how selective perforating and selective fracturing gave much better fracturing coverage of an Eocene formation. This formation is a highly lenticular zone, but it is somewhat continuous.

When perforations were close together, ball sealers were not effective in getting the induced fracture to cover all the productive intervals. When perforated intervals were spaced 30 m apart, staging with perforation ball sealers was effective in getting the fracture to communicate and stimulate all sands.

Stimulation of the Cretaceous age formation was also covered by Mr Von Albrecht. This was a lenticular type formation also; however, entirely different techniques were required. The were only five or six widely spaced zones. Laboratory evaluations showed an acid external emulsion gave the greatest stimulation and field results agreed. As in the Eocene, staged treatments were most effective.

Another problem in this reservoir was asphaltene deposition. Asphaltenes and paraffins came out of solution under some producing conditions. Temperature and dissolved gas had a great influence as to where these came out of solution and were deposited. Flowing wellhead temperatures were increased and dissolved gas content was decreased to remove the deposition region from the well. Wellhead temperature was increased by increasing flowing rates, often through stimulation or

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