The 3.1- and 3.7-km-deep FFC-1 and DGE-1 geothermal explorations wells drilled into the Precambrian crystalline basement on the southern margin of the Fennoscandian Shield are evaluated regarding experiences from drilling, geological conditions, and thermal properties. Both wells penetrate an approximately 2-km-thick succession of sedimentary strata before entering the crystalline basement, dominated by orthogneiss, metabasite and amphibolite of the (1.1–0.9 Ga) Eastern Interior Sveconorwegian Province. The upper c. 400 m of the basement is in FFC-1 severely fractured and water-bearing which disqualified the use of percussion air drilling and conventional rotary drilling was, therefore, performed for the rest of the borehole. The evaluation of the rotary drillings in FFC-1 and DGE-1 showed that the average bit life was very similar, 62 m and 68 m, respectively. Similarly, the average ROP varied between 2 and 4 m/h without any preferences regarding bit-type (PDC or TCI) or geology. A bottomhole temperature of 84.1 °C was measured in FFC-1 borehole with gradients varying between 17.4 and 23.5 °C/km for the main part of the borehole. The calculated heat flow varies between 51 and 66 mW/m2 and the average heat production is 3.0 µW/m3. The basement in FFC-1 is, overall, depleted in uranium and thorium in comparison to DGE-1 where the heat productivity is overall higher with an average of 5.8 µW/m3. The spatial distribution of fractures was successfully mapped using borehole imaging logs in FFC-1 and shows a dominance of N–S oriented open fractures, a fracture frequency varying between 0.85 and 2.49 frac/m and a fracture volumetric density between 1.68 and 3.39 m2/m3. The evaluation of the two boreholes provides insight and new empirical data on the thermal properties and fracturing of the concealed crystalline basement in the Fennoscandian Shield Border Zone that, previously, had only been assessed by assumptions and modelling. The outcome of the drilling operation has also provided insight regarding the drilling performance in the basement and statistical data on various drill bits used. The knowledge gained is important in feasibility studies of deep geothermal projects in the crystalline basement in south Sweden.

中文翻译：

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瑞典南部芬诺斯坎迪亚地盾边界区结晶基底深部地热勘探钻探评价

对在 Fennoscandian 地盾南缘前寒武纪结晶基底中钻探的 3.1 和 3.7 公里深的 FFC-1 和 DGE-1 地热勘探井进行了钻探经验、地质条件和热特性的评估。两口井在进入结晶基底之前都穿透了大约 2 公里厚的沉积地层序列，主要是 (1.1-0.9 Ga) 东内陆斯韦科诺韦吉亚省的正方麻岩、变贝岩和角闪岩。上层 C. 400 m 的地下室位于 FFC-1 中，严重裂缝和含水，不符合使用冲击空气钻井的资格，因此，对其余钻孔进行了常规旋转钻井。对 FFC-1 和 DGE-1 中旋转钻孔的评估表明，平均钻头寿命非常相似，分别为 62 m 和 68 m，分别。同样，平均 ROP 在 2 到 4 m/h 之间变化，对钻头类型（PDC 或 TCI）或地质没有任何偏好。在 FFC-1 钻孔中测得的井底温度为 84.1 °C，钻孔主要部分的梯度在 17.4 至 23.5 °C/km 之间变化。计算出的热流在 51 到 66 mW/m2 之间变化，平均产热量为 3.0 µW/m3。与 DGE-1 相比，FFC-1 中的地下室总体上贫铀和钍，DGE-1 的热生产率总体较高，平均为 5.8 µW/m3。使用 FFC-1 中的钻孔成像测井成功绘制了裂缝的空间分布，显示 N-S 向的开放裂缝占主导地位，裂缝频率在 0.85 和 2.49 frac/m 之间变化，裂缝体积密度在 1.68 和 3.39 m2/之间立方米。对两个钻孔的评估提供了关于 Fennoscandian 地盾边界区隐蔽结晶基底的热性能和破裂的洞察力和新的经验数据，以前，这些数据只能通过假设和建模进行评估。钻井作业的结果还提供了有关地下室钻井性能和使用的各种钻头的统计数据的见解。所获得的知识对于瑞典南部水晶地下室深层地热项目的可行性研究非常重要。钻井作业的结果还提供了有关地下室钻井性能和使用的各种钻头的统计数据的见解。所获得的知识对于瑞典南部水晶地下室深层地热项目的可行性研究非常重要。钻井作业的结果还提供了有关地下室钻井性能和使用的各种钻头的统计数据的见解。所获得的知识对于瑞典南部水晶地下室深层地热项目的可行性研究非常重要。