This paper describes the results of formation damage study on first horizontal well drilled and completed in Croatia. The specific problem of formation damage caused by polymer mud was evaluated in the laboratory, investigating the rock-fluid compatibility and methods for restoring the reservoir permeability around the wellbore. The source of damage was identified as a vey tough mud filtrate cake as well as the reduction of permeability inside the pore space due to mud filtrate invasion. The two stages well stimulating procedure for damage removal, consisting of soaking and injecting oxidizer, then acid, in two steps, tested in the laboratory, showed promising results.


In the last decade the technology of horizontal wells,1 because of their enhanced productivity, has become widespread. More than 2000 horizontal wells are completed each year worldwide.

Maintaining maximum deliverability from these wells requires that formation damage be minimized. In turn, minimizing or preventing formation damage, which is either created during drilling or removed during well completion and production, requires serious in-depth research.

Horizontal wells are usually completed by either setting the predrilled or slotted uncemented liner, or leaving the horizontal section as an open hole. In both cases the production interval is not perforated. This means that the horizontal well lacks the perforation tunnels that are normal used in conventional vertical wells to bypass part of the damage caused by drilling mud invasion.

Mud damage, which adversely affects productivity, occurs as a result of several factors. First, in open hole, the filter cake2, can be formed both as external and uniform at the formation face and as internal, inside the porous medium around the wellbore. Another problem is the permeability reduction that occurs because of the interaction between the rock and incompatible mud filtrate that penetrates into the vicinity of the wellbore. These two factors, often combined with the heterogeneity of the payzone3 compound the risk of impaired productivity in horizontal wells.

Very often for horizontal wells, the water base polymer-type systems are used as drilling fluids. By their nature, polymers contained in the filter cake4,5 are tough and not easy degradable. Moreover, the complexities involved in the study of mud filtrate and rock interactions necessitate more detailed research to identify and control the formation damage in horizontal wells.

The most optimistic approaches to this problem are to select of mud system that is as unintrusive as possible, or, because a damage is practically inevitable, to implement a proper damage removal, well stimulation method. Well treatment may include filter cake clean up (using drawdown pressure to initialize production5, and/or application of a chemical breaker system to destabilize and remove the filter cake)5,6 and/or chemical stimulation targeted toward mud filtrate damage elimination.

The findings that relate to problems of productivity impairment due to formation damage of the first horizontal well drilled and completed in Croatia are described in this paper.

Case History

Well A7 has been designed as a high rate gas-condensate producer and was drilled into the reservoir inside the already developed field, located in the southern part of Pannonian Basin - the Sava Depression. The well parameters are shown in Table 1. The targeted reservoir was at an average depth of 2400 m (TVD), and has an effective thickness of 29 m. The gas-bearing rock is low to medium compact sandstone (mineral composition is shown in Table 2). According to lithological data from surrounding wells, the reservoir lithology is characterized as laminated sandstone intersected with shale layers, which indicated petrophysical anisotropy as a result of complex sedimentary genesis.

After completion, before bringing the well onto production, the slotted liner was briefly cleaned up with 2% HCl through 2 3/8 in. tubing lowered to 2890 m in an attempt to disintegrate and remove the mud filter cake.

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