Successful Stimulation Practices Offshore Holland
Authors: Coulter Gerald R. , Purvis S. Barrie 
Published: December 2007

Summary
Analyses of the Rotliegendes sandstone indicate this formation offshore Holland contains clay minerals of various types, making it somewhat sensitive to water-base stimulation fluids. Both hydrochloric (HCl) and hydrofluoric (HF) acids have been used successfully for acid wash and/or mud damage removal. For hydraulic fracturing, water-base fluids and emulsions are used successfully. Laboratory analyses of core samples assist in determining the most appropriate fluid for treatment. Testing strings have been used, allowing proper zone testing and stimulation through the string if required. Surface equipment requirements for stimulation have been designed around the drilling rig's space and weight limitations.

Introduction
Offshore Holland has been an active exploration and producing area for several years. The zone of primary interest has been the Rotliegendes sandstone of Lower Permian age. Almost all commercial field development has been in this formation, and it is the subject of this paper.The Rotliegendes sands offshore Holland generally have low permeability, requiring stimulation for commercial production rates. Primarily, these sands are gas bearing; however, liquid hydrocarbons have been discovered in some cases. Depth to the top of the Rotliegendes is approximately 9,840 ft (3000 m). The bottomhole temperature (BHT) and pressure (BHP) are approximately 230F (100C) and 4,750 psi (32.8 MPa), respectively. Considerable laboratory testing of core samples has been carried out to evaluate the effect of various stimulation fluids on the formation. A summary of this information is presented. Because most ofthis stimulation work is conducted from jackup drilling rigs, special consideration has been given to formation testing tools and testing strings. The tool strings are designed to allow proper testing of the zones of interest and, if necessary, stimulation through the string. Examples of tool strings are presented.Also of concern are space availability and weight restrictions on these rigs for stimulation equipment. For hydraulic fracturing, normally 2,000 to 3,500 hhp (2611 kW) is required, with additional supporting equipment.Fig. 1 shows a general view of the North Sea area and Fig. 2 shows the offshore Holland area in more detail.

Stimulation Considerations
The Rotliegendes formation is primarily a nonmarine sandstone with four identifiable facies. These facies vary from dune deposits, wadi deposits, interdune sabkha, and fluvial deposits. The reservoir quality of the rock may be related to each of these facies.The Rotliegendes zones considered for wellbore cleanup and/or stimulation vary little in mineralogical composition. A generalized X-ray diffraction analysis of Rotliegendes core samples is shown in Table 1. Examples of the Rotliegendes pore-associated minerals are shown in the scanning electron microscope (SEM) views, Figs. 3, 4, and 5. Fig. 3 is a split photo showing the type of Rotliegendes where abundant illite clay is present. Figs. 4 and 5 show a section of Rotliegendes where, at the higher power magnification, very little illite is seen extending into the pore space.Laboratory tests have been conducted to assist in selection of appropriate fluids for wellbore cleanup and/or stimulation of the Rotliegendes. In general, the laboratory tests evaluating wellbore cleanup/mud damage removal indicate that in very low permeability Rotliegendes cores (&lt;0.1 md) removal of matrix damage is difficult with either HCl or HF acid. However, perforation soaks with 7 1/2 to 15% HCl acid have been carried out successfully in these low permeability zones.
JPT
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